chore: Drop yarn2nix and switch to pnpm.

This is a wide-ranging change in which we drop `yarn2nix` (and Yarn
itself) in favor using `pnpm` directly, with no Nix layer in-between
the `pnpm` lockfile and the packages we use for builds. This means
we're giving up on most of what Nix provides -- here we only use it to
ensure that everyone is using the same version of `pnpm`, sourced from
`nipxkgs` via our `nix develop` shell.

I'm doing this reluctantly, but `yarn2nix` is falling further and
further behind the Nodejs ecosystem, it always feels like we're just
one major package version bump away from everything breaking, and
there's no viable replacement on the horizon for `yarn2nix` that I can
see. Nix is not really buying us much in this repo to begin with, so
we don't lose that much with this change. Hopefully this leads to
better DX going forward overall.

Note that the `README.md` is heavily edited to replace all the `yarn`
commands with the equivalent `pnpm` commands (where they exist; we
have lost some functionality by dropping Yarn, see below), and I
wouldn't be surprised if there are some discrepancies in the
documentation vs the reality. Please file an issue if you notice any.

A few more items of note:

* I have bumped our `nixpkgs` pin here, but only to get a more recent
version of `pnpm`. I have not bumped any Nodejs package versions.

* `pnpm` is more strict about dependencies, so I've had to add a few
that Yarn was magically finding for us.

* We've lost the nice `wsrun watch` and `wsrun watch:storybook`
commands that would run the entire project in `watch` (HMR) mode with
a single command. This is because `wsrun` is designed to work with
Yarn workspaces only, and doesn't work with `pnpm` workspaces. As far
as I can tell, there's no equivalent to `wsrun` for `pnpm`, which
means that if you want the run the frontend or our Storybook in HMR
mode and automatically pick up changes made to the other packages in
the repo upon which the frontend/Storybook depend, you'll need to open
multiple shells and run a `pnpm watch` command in each one,
simultaneously.

* We do not implement the push-to-Chromatic CI step here, as I want to
test just building the app before we get into trying to build
Storybook in CI. (However, I have been able to successfully build and
use Storybook in my local shell with these changes.)

* Speaking of Storybook, we have to do some ugly hacks to make it work
with `pnpm`. Many people have trouble with this and there are a litany
of issues, many with different workarounds that are sometimes
successful, sometimes not; see:

pnpm/pnpm#4268
storybookjs/storybook#13428
https://github.com/storybookjs/builder-vite/blob/782261437b3cb3e2f3d013cd37f1faa14863f205/packages/builder-vite/README.md#installation
storybookjs/builder-vite#55
storybookjs/builder-vite#237
https://github.com/IanVS/vite-storybook-pnpm-shame
https://github.com/jdk2pq/vite-storybook-pnpm-vue

Until Storybook fix their various issues, this looks like the proper
"fix", but it will require quite a bit of work, I suspect:

pnpm/pnpm#4268 (comment)

However, in the interest of time, I've chosen this option, which is
the least bad workaround I've seen that worked for me:

storybookjs/storybook#13428 (comment)

.buildkite/pipeline.yaml

@@ -25,16 +25,3 @@ steps:
       nix develop --print-build-logs --profile /tmp/primer-app --command cachix --verbose --config "$CACHIX_CONFIG" push "$CACHIX_CACHE" /tmp/primer-app
     agents:
       os: "darwin"
-  - label: ":chromatic: Trigger Chromatic deployment"
-    trigger: primer-app-chromatic
-    # This can't be async, because we need to be sure the Chromatic
-    # deployment succeeded before merging via Bors.
-    async: false
-    build:
-      message: "${BUILDKITE_MESSAGE}"
-      commit: "${BUILDKITE_COMMIT}"
-      branch: "${BUILDKITE_BRANCH}"
-      env:
-        BUILDKITE_PULL_REQUEST: "${BUILDKITE_PULL_REQUEST}"
-        BUILDKITE_PULL_REQUEST_BASE_BRANCH: "${BUILDKITE_PULL_REQUEST_BASE_BRANCH}"
-        BUILDKITE_PULL_REQUEST_REPO: "${BUILDKITE_PULL_REQUEST_REPO}"

.github/workflows/build-primer-app.yaml

@@ -0,0 +1,40 @@
+name: Build the frontend
+
+on:
+  push:
+    branches:
+      - main
+      - gh-readonly-queue/main/*
+  pull_request:
+
+jobs:
+  build:
+    permissions:
+      contents: read
+      id-token: write
+      deployments: write
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v3
+        with:
+          # Required by the Chromatic GitHub Action
+          fetch-depth: 0
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v2.2.1
+        with:
+          version: 7.1.0
+
+      - name: Setup nodejs
+        uses: actions/setup-node@v3.2.0
+        with:
+          node-version: 14
+          cache: "pnpm"
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Build the frontend
+        run: pnpm build

.gitignore

@@ -21,10 +21,10 @@ tsconfig.tsbuildinfo
 # tsserver
 .log/
 
-# yarn
+# dependencies
 node_modules/
-yarn-debug.log*
-yarn-error.log*
+/.pnp
+.pnp.js
 
 # vite
 .vite/
@@ -40,6 +40,3 @@ build-storybook.log
 # VSCode
 *.code-workspace
 .vscode
-
-# Ultra
-.ultra.cache.json

.npmrc

@@ -0,0 +1,5 @@
+engine-strict=true
+auto-install-peers=true
+public-hoist-pattern[]=*storybook*
+public-hoist-pattern[]=*react*
+public-hoist-pattern[]=*babel*

.prettierignore

@@ -1,7 +1,7 @@
 README.md
 node_modules/
 dist/
-yarn.lock
+pnpm-lock.yaml
 .direnv/
 .log
 result*