support TypeScript-format config

[g-plane]

Changes

Added support for TypeScript-format config, so the config file can be snowpack.config.ts now, which allow config file to be type-checked.

Testing

Added a new "build" test for this change. Snapshots are generated.

Docs

Added mention about supporting TypeScript-format config at "reference" section.

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[FredKSchott]

strong +1 on this feature request, but I think this implementation will break any relative imports (../ out of the temp dir is going to point somewhere else).

I'm not sure of a better way to do this though. We could create the '.snowpack.config.cjs' file alongside snowpack.config.ts temporarily, and then delete it when we're done. It sounds like cosmicconfig does some caching, which means this wouldn't have to happen every time. That's the best I can come up with right now

[g-plane]

Outdated. Please read next comment.

I've re-implemented this part (see latest commit). The new way is using Node.js built-in vm module and execute it. Because the code is wrapped in a function, variables created in the config file won't be leaked into outer scope. And, there won't be filesystem operations.

[g-plane]

Now the config file will be bundled by esbuild. That means any imports will be bundled into a single file, so placing the compiled file in temp directory should be safe now.

[FredKSchott]

That's clever! It's definitely a solution to the relative imports problem but introduces a lot of extra effort with potential for slow down and bugs. I wouldn't feel comfortable unless this also came with a console.warn("TypeScript config support is still experimental").

tbh I still prefer the temporary single-file method outlined in https://github.com/pikapkg/snowpack/pull/1253/files#r501834625 to bundling the entire config. A light touch for a workaround like this is valuable. And, since cosmicconfig does caching, the users should rarely notice it.

[g-plane]

While we can put the temporary file into current working directory, I think bundling is necessary, because this allows to import other TS files.

[g-plane]

ping @FredKSchott

[FredKSchott]

still thinking about this. St the very least, we want to get the current release that's on master out first, and then revisit this post-release. Hopefully getting that out tonight/tomorrow.

[FredKSchott]

Also, tests were red on master overnight. Rebasing on master should get tests passing.
Also for the merge conflict that you're seeing, "docs/docs/10-reference.md" is now at "docs/10-reference.md"

[FredKSchott]

Okay, I've thought about this some more and am +1 on merging, with the caveat of adding a special error around the bundling and require-ing of a TS config file. Basically, something to the effect of:

try {
  const ... = esbuild.build()
  const ... = require()
  return ...
} catch (err) {
  logger.error('Warning: TypeScript config file support is still experimental. Consider moving to JavaScript if you continue to have problems');
  throw err;
}

@g-plane thanks again for your patience here! If you can rebase this PR and add this fix, I'll +1 and merge in time for the next v2.15 release.

[FredKSchott]

LGTM! Had to remove the docs mention for now since this won't go out immediately (and our docs site does release immediately) but thank you for adding that.