docs: add API extractor

[grant]

Add setup for creating API docs for the Node Functions Framework. Does not attempt to annotate whole library (but just enough to make the generator not err).

Fixes #349

Generated API docs: https://github.com/GoogleCloudPlatform/functions-framework-nodejs/blob/grant_api_extractor/docs/api.md

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Note: There aren't that many types right now because we removed a large chunk of the exported API: https://github.com/GoogleCloudPlatform/functions-framework-nodejs/pull/347/files#r724614498

I'm not sure though.

I think we're trying to add that back before releasing a new version of this framework.

[matthewrobertson]

There aren't that many types right now because we removed a large chunk of the exported API

Something is fishy here, but I don't think we removed the exported API. I see some issues that suggest api-extractor has trouble with re-export statements: microsoft/rushstack#663

Maybe we need to just update this line in src/index.ts

[matthewrobertson]

@grant I investigated a bit I think you were having problems because you were adding comments to things we were re-exporting in index.ts. I think when you do that it clobbers any annotations in the underlying file.

[grant]

Thanks @matthewrobertson @anniefu, I updated the generated docs. They should be a bit more comprehensive now too :) PTAL

[matthewrobertson]

I think we eventually want to actually make this the types we use in our package.json file.

[grant]

Sure, that's fine. There would need to be some script or tooling to update this generated file with package.json types.

Or we could simply not generate this file.

[matthewrobertson]

LGTM. Please fix the warning before landing.

As a next step it would be good to add an action to automatically validate or generate these when a PR gets submitted.