Add support for TS entry points in monorepo configuration

[Unnvaldr]

Firstly, thank you @efokschaner and @Gerrit0 for implementing packages entry points 😃

I've added it as we talked about it in #1567, for now TS files must have extension set explicitly, otherwise they won't be "fetched".

If it's fine as-is, it would be nice to get it released in beta channel ASAP.

[efokschaner]

Thanks for contributing!

[Unnvaldr]

CONTINUATION OF DISCUSSION

Should we first allow the default value of index.js to be tested for existence before we resort to the types field?
I could imagine a package which had an index.ts, an index.js, and index.d.ts, and therefore doesn't specify package.main,

With this change typedoc would now parse index.d.ts for docs rather than index.ts, while it's probably preferable to parse index.ts.

On the other hand, usually when main & types are specified, it means that dev has specified a separate file created for storing his types definitions (case when your project is written in JS, but you've added TS types only to not rewrite whole project in TS), so logically we should check out types/typings first. Previously we had includeDeclarations flag for controlling whenever you wanted to include .d.ts files in documenting process, but AFAIK it's gone since v0.20 (which would be helpful to disable/enable behaviour described above).

[efokschaner]

When main is specified (or the default works) we use the source mapping in the js to find the original ts. If we can't find the .ts I agree with checking for types (the .d.ts).
We should prefer finding the .ts though as it's more likely to contain the kind of tsdoc markup + other annotations that power typedoc, not to mention creating links to the .ts rather than the .d.ts in typedoc output.

So, in short, I agree with using the .d.ts when there is no .ts, as long as we try to find that .ts first.

[Gerrit0]

This seems reasonable to me, thanks!