add type-fest user test

[voxpelli]

In the type-fest type library there are a lot of complex usages of types and as such it often finds edge cases not seen elsewhere.

I have tried to have a canary test there and will have again when this gets merged: sindresorhus/type-fest#785

But even when running that frequently against nightlies it can be hard to pinpoint what it was that caused something to break, as seen here: sindresorhus/type-fest#784

I personally would find it great if type-fest could be tested here, especially so as its relied upon by many well-used module, resulting in +151 million weekly downloads from npm.

That way we can more early discover if there's an incompatibility and aid in helping out on solving it.

/ Pelle, co-maintainer of type-fest

[jakebailey]

Looks good, though I really suspect that type-fest is already a part of the topXYZ test suite, which tests the top TS repos.