refactor: migrate project to Typescript

[JKRhb]

This PR is a potential follow-up to #289 migrating the whole project to Typescript.

[coveralls]

Pull Request Test Coverage Report for Build 1427551635

• 255 of 262 (97.33%) changed or added relevant lines in 15 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 87.444%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/server.ts	37	38	97.37%
index.ts	56	59	94.92%
lib/cache.ts	37	40	92.5%
<!--	Total:	255	262	97.33%	-->

Totals
Change from base Build 1427500654:	-0.5%
Covered Lines:	1131
Relevant Lines:	1244

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💛 - Coveralls

[Apollon77]

wow, now you go forward :-))

[invaderb]

This is awesome! great job so far :)

[JKRhb]

@invaderb @Apollon77 Thank you for your feedback! :) I now migrated most of the code, there are still some issues, though, which I have to investigate. I guess the PR should be ready for review later this week :)

[JKRhb]

After some intense procrastination debugging, I now managed to finish this PR and move the whole codebase to Typescript :) Feedback is highly appreciated!

Both tests and the linter currently fail here on GitHub because https://github.com/DefinitelyTyped/DefinitelyTyped/pull/56130 hasn't been merged yet but once that is done it should be possible to merge this PR as well. I guess #289 has become obsolete now or do you think it might make sense to have some kind of intermediary step between the old state of the project and this PR?

To better keep track of the changes I'll might create a couple of smaller PRs from this one that introduce certain aspects (like the null checks or the use of Maps instead of Objects for caching) first and then finally merge the rest of this PR.

[JKRhb]

After the merge of https://github.com/DefinitelyTyped/DefinitelyTyped/pull/56130, the tests are now finally passing :)

[JKRhb]

Ehem, sorry for all the updates. I think this PR should now be finished :) After the release of 0.26.0 we should be able to merge it right away and release 1.0.0.

[Apollon77]

seems a rebase is needed @JKRhb

[JKRhb]

seems a rebase is needed @JKRhb

Done :)