Type test all files in itwinui-react

[r100-stack]

From #1514 (comment).
Related threads: #1514 (comment), #1514 (comment)

Instead of having tsc in build just type check the files that will be built, we should type check all files. By all files, I mean even the unit test files.

This is because unit tests currently pass even with type errors in them. So there is no way of detecting some very specific type error if it only occurs in the unit test files.

[mayank99]

Folding into #1571 for ease of tracking.