Add StringRepeat type

[Max10240]

Closes #827

I saw that the issue has a help wanted tag, and I have some free time, so I finished it.
Note that I renamed it to StringRepeat instead of RepeatString to make it compatible with StringSlice...

[sindresorhus]

This would also be a good candidate for https://github.com/sindresorhus/ts-extras

[Max10240]

This would also be a good candidate for https://github.com/sindresorhus/ts-extras

A new function like stringRepeat ? Being a JS function doesn't seem to make much sense IMO(Although that's the example I wrote in the annotation document of type RepeatString😅, I think it's probably not actually very common in the JS world), I guess few people care about the exact type of value returned by the function stringRepeat. Please let me know if I misunderstand anything.

[sindresorhus]

Yeah, thinking more about it, I cannot come up with a situation where I would actually need the literal string as a type when doing actual TS.