feat: add shareable tsconfig support

[p-chan]

I used get-tsconfig to support tsconfig extends property. Fix https://github.com/danger/danger-js/issues/1283

[fbartho]

This PR looks like it’s a duplicate to #1313 — and that one has tests and an approach that better matches my feedback above.

Not sure why that one stalled out though! So you might be able to pick up from where they started, or at least use it as a hint.