Shallow merge in `tsconfigOverride` where TS does so -- match `tsconfig` `extends`

[ezolenko]

What happens and why it is wrong

typescript is using shallow merge on some elements (paths, possibly includes etc). Need to make plugin behave the same way to reduce surprises.

#72

[agilgur5]

Per the TS docs on extends -- which we should treat as a "source of truth" -- the fields that should be shallow merged (replaced, rather) are files, include, and exclude.

Basically all the top-level arrays. paths isn't mentioned. It's possible that's just missing docs though, but I haven't checked the TS API to confirm which it does. Also references specifically isn't inherited, so not sure if we want to have the same behavior for that one or not.