Introduce IDE-only tsconfig.json to check tests too.

[phryneas]

This creates a new tsconfig.json in src that will be picked up by VSCode and apply the rules of the parent tsconfig.json to tests as well - without tooling that relies on the <root>/tsconfig.json (e.g. the build steps) having to pick up all the test files.

It should also fix the matcher inclusion problem (toRerender in expect(ProfiledHook).not.toRerender(); being unknown unless another editor window has the matches/index.d.ts open).

I'm opening this PR against the strict: true PR as that PR fixes a lot of problems we'd otherwise see in our tests.

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[phryneas]

Oh great, for some cursed reason this crashes our linter. fixed it.

[jerelmiller]

🎉 thanks for looking into this!

[alessbell]

+1, thank you for fixing this!