Perform TypeScript type checking while bundle-serving

[afoxman]

This is related to #113. The difference being that I want typescript validation while running a metro bundle server. As source code is updated, metro detects the changes, transpiles, and publishes delta-bundles through fast-refresh. I want any added/modified files to be transpiled (removed files will be ignored).

The output should show up in the metro bundle-server log console.

Type errors should cause the fast-refresh bundle generation to fail, if possible, leaving the running app unchanged.

[afoxman]

As this is meant to be a long-running process, it should use tsserver to benefit from caching. This means it will have a substantially different internal architecture than the code in #113.

Initial idea is to use a new experimental hook in the metro config: experimentalSerializerHook. It is supposed to be called anytime the graph changes. That's where I'll start. Then I'll move on to seeing how I can output to the metro log console, and how I can fail the bundle generation process.

[afoxman]

Additional use-case to consider: when a metro server is running, it should react to changes in any dependent package in the repo, not just the app package. If it doesn't make that work because we need it. Once Metro is watching all in-repo dependencies, make sure that typescript validation is happening on these dependencies -- specifically, on the source .ts[x] files.

[afoxman]

Made a successful prototype using Metro's experimentalSerializerHook which provide delta updates. Works during bundling and serving. Catches changes in the current package and in external packages.

Branch: afoxman/serializer-hook-prototype

[afoxman]

Another step -- I updated Metro so that calls to experimentalSerializerHook now deliver the target platform as well. We're going to need this when telling TypeScript how to type-check any files that were added/updated.

https://github.com/afoxman/metro/tree/add-platform-to-serializer-hook

Using Verdaccio as a local npm registry, I was able to publish this build (and consumers: react-native, @react-native-community/cli, @rnx-kit) and verify that the entire pipeline is working end-to-end.