Fix TypeScript types

[mmkal]

Fixes #84

First commit a3b3bad adds typescript, a tsconfig.json file and switches to from tsd to expect-type (due to tsdjs/tsd#142), to expose the problem in the test-d file.
Second commit will fix the bug in the typings.

[mmkal]

@sindresorhus if you run the CI workflow we should see a ❌ , then I can push the fix so it can become a ✅

[mmkal]

Note: it might be possible to workaround the tsd issue. The error messages are a bit better in CI builds. Feel free to close this if you'd prefer to figure that out.

[sindresorhus]

I would prefer using tsd where it works, but I'm ok with using expect-type for the assertions that are borked in tsd.

[mmkal]

@sindresorhus as far as I can tell, they are borked for any generic function. Borked in the sense that they don’t catch errors. So even though tsd only fails to catch the bug in dot-prop types in a couple of assertions, if the bug were more serious it would fail on the others. So at least for this library I don’t think it’s safe to use.

[sindresorhus]

Alright. Let's use expect-type.

[mmkal]

@sindresorhus worth noting - when unsure what the return type would be, it defaults to undefined. The "default" in type-fest.Get is unknown, since typescript in general allows for additional properties, so we can't know for sure it's undefined. It seems that getProperty effectively overrides the behaviour by making the default DefaultValue undefined - IMO it shouldn't.

I want to keep this PR focused so I'm not proposing to change that here but I think it should be considered to switch to unknown (obviously best solution is to make Get respect \. but that would make it incompatible with lodash.get so 🤷 )

[sindresorhus]

I agree: #95

[sindresorhus]

Thanks for fixing :)