feat: dual tsc builds trying to address microsoft/TypeScript#49462

.eslintrc.json

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+{
+  "env": {
+    "es6": true,
+    "node": true
+  },
+  "extends": [
+    "eslint:recommended",
+    "plugin:n/recommended"
+  ],
+  "parserOptions": {
+    "ecmaVersion": 2023,
+    "sourceType": "module"
+  },
+  "rules": {
+    "no-console": "error",
+    "n/shebang": [
+      "error",
+      {
+        "convertPath": {
+          "src/*.js": ["^src/(.+)$", "dist/$1"]
+        }
+      }
+    ]
+  }
+}

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  ci:
+    name: CI
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3.5.2
+      - name: Setup Node
+        uses: actions/setup-node@v3.6.0
+        with:
+          node-version: '20.5.0'
+      - name: Install Dependencies
+        run: npm ci
+      - name: Save error log
+        uses: actions/upload-artifact@v2
+        if: ${{ failure() }}
+        with:
+          name: npm-debug-log-${{ hashFiles('package-lock.json') }}
+          path: npm-debug.log
+      - name: Lint
+        run: npm run lint
+      - name: Test
+        run: npm test
+      - name: Report Coverage
+        uses: codecov/codecov-action@v3.1.4
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+      - name: Pack
+        run: npm pack

.github/workflows/publish.yml

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+name: Publish
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  publish:
+    if: contains('["knightedcodemonkey"]', github.actor)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3.5.2
+      - name: Setup Node
+        uses: actions/setup-node@v3.6.0
+        with:
+          node-version: '20.5.0'
+      - name: Install Dependencies
+        run: npm ci
+      - name: Save error log
+        uses: actions/upload-artifact@v2
+        if: ${{ failure() }}
+        with:
+          name: npm-debug-log-${{ hashFiles('package-lock.json') }}
+          path: npm-debug.log
+      - name: Lint
+        run: npm run lint
+      - name: Test
+        run: npm test
+      - name: Pack
+        run: npm pack
+      - name: Push to NPM registry
+        uses: JS-DevTools/npm-publish@v2.1.0
+        with:
+          token: ${{ secrets.NPM_AUTH_TOKEN }}
+          access: public

.gitignore

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+*.tgz
+dist
+node_modules
+coverage

README.md

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+# [`@knighted/duel`](https://www.npmjs.com/package/@knighted/duel)
+
+![CI](https://github.com/knightedcodemonkey/duel/actions/workflows/ci.yml/badge.svg)
+[![NPM version](https://img.shields.io/npm/v/@knighted/duel.svg)](https://www.npmjs.com/package/@knighted/duel)
+
+Node.js tool for creating a TypeScript dual package.
+
+Early stages of development. Inspired by https://github.com/microsoft/TypeScript/issues/49462.
+
+## Example
+
+Consider a project that is ESM-first, i.e. `"type": "module"` in package.json, that also wants to create a separate CJS build. It might have a tsconfig.json file that looks like the following.
+
+**tsconfig.json**
+
+```json
+{
+  "compilerOptions": {
+    "target": "ESNext",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "declaration": true,
+    "strict": true,
+    "outDir": "dist"
+  },
+  "include": ["src/*.ts"]
+}
+```
+
+Running the following will use the tsconfig.json defined above and create a separate CJS build in `dist/cjs`.
+
+```console
+user@comp ~ $ duel -p tsconfig.json -x .cjs
+```
+
+Now you can update your `exports` in package.json to match the build output.
+
+It should work similarly for a CJS first project. Except, your tsconfig.json would be slightly different and you'd want to pass `-x .mjs`.
+
+## Gotchas
+
+Unfortunately, TypeScript doesn't really understand dual packages very well. For instance, it will **always** create CJS exports when `--module commonjs` is used, even on files with an `.mts` extension. One reference issue is https://github.com/microsoft/TypeScript/issues/54573.

babel.config.json

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+{
+  "targets": "node >= 16.19",
+  "presets": [
+    ["@babel/preset-env", {
+      "modules": false
+    }]
+  ]
+}

codecov.yml

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+coverage:
+  status:
+    project:
+      default:
+        target: 80.0
+        threshold: 0.5
+    patch:
+      default:
+        target: 80.0
+        threshold: 1.0