refactor: combine `check-tsconfig` with `parse-tsconfig`

[agilgur5]

~NOTE: this is built on top of #404 as it uses the context changes made there. As such, I've marked this PR as "Draft" until #404 is merged.~ Rebased on top and marked as ready for review.

Summary

Combine the two tsconfig files into one as the split is not currently necessary and adds unnecessary complexity

• Follow-up to #396

Details

• checkTsConfig is literally only used by parseTsConfig

  • I first just moved the function over, but it's a two-liner, so thought it made more sense to just in-line it

• merge the unit tests as well

  • we already check the non-error case in parse-tsconfig.spec, so only add the error case

• check-tsconfig was longer in the past and more files were being created then for separation, so this may have made more sense then, but now it is unnecessary

  • c.f. 733207791800ffe15672554d2a6337c73c65cf74

[agilgur5]

This is gonna merge conflict with #404 -- please merge that first as it's a complicated one that a bunch of other code depends on ~EDIT: nvm, see below~ EDIT2: more of a "silent" merge conflict due to variable renaming, so will build this on top of #404 after all and fix the renaming issue. Sorry for any notification churn!