deps: Add typescript as dev dependency.

We don't need TypeScript ourselves [1], but we want to upgrade
`eslint-plugin-jest` soon, and a rather distant dependency has a
peer dependency of TypeScript (as we also saw in 01593b3). Here's
the output:

```
warning "eslint-plugin-jest > @typescript-eslint/experimental-utils
> @typescript-eslint/typescript-estree > tsutils@3.17.1" has unmet
peer dependency "typescript@>=2.8.0 || >= 3.2.0-dev || >= 3.3.0-dev
|| >= 3.4.0-dev || >= 3.5.0-dev || >= 3.6.0-dev || >= 3.6.0-beta ||
>= 3.7.0-dev || >= 3.7.0-beta".
```

Ah well, at least we can put it in `devDependencies`. And `yarn why
typescript` reveals that `prettier-eslint` and `prettier-eslint-cli`
have both already been bringing it in as their dependency.

[1] zulip#3458 is open to consider migrating from Flow to TypeScript, but
    that's not what this commit is about.

package.json

@@ -130,6 +130,7 @@
     "react-native-cli": "^2.0.1",
     "redux-mock-store": "^1.5.1",
     "rollup": "^2.26.5",
+    "typescript": "^4.0.2",
     "yarn-deduplicate": "^1.1.1"
   }
 }

yarn.lock

@@ -9816,6 +9816,11 @@ typescript@^3.2.1, typescript@^3.4, typescript@^3.9.3:
   resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.9.7.tgz#98d600a5ebdc38f40cb277522f12dc800e9e25fa"
   integrity sha512-BLbiRkiBzAwsjut4x/dsibSTB6yWpwT5qWmC2OfuCg3GgVQCSgMs4vEctYPhsaGtd0AeuuHMkjZ2h2WG8MSzRw==
 
+typescript@^4.0.2:
+  version "4.0.2"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-4.0.2.tgz#7ea7c88777c723c681e33bf7988be5d008d05ac2"
+  integrity sha512-e4ERvRV2wb+rRZ/IQeb3jm2VxBsirQLpQhdxplZ2MEzGvDkkMmPglecnNDfSUBivMjP93vRbngYYDQqQ/78bcQ==
+
 ua-parser-js@^0.7.18:
   version "0.7.21"
   resolved "https://registry.yarnpkg.com/ua-parser-js/-/ua-parser-js-0.7.21.tgz#853cf9ce93f642f67174273cc34565ae6f308777"