Add a file provider that avoids ts program walk

[Adjective-Object]

Adds a faster file provider that discovers source and fence files with a
fast asyncronous file walk rather than by relying on typescript to
perform a syncronous walk + parse of the program.

We still use typescript to parse compilerOptions, but since we no longer
rely on the typescript program walk to identify files, we stub out file
discovery so only the initial config parse happens.

Also adds config options to support switching between the two providers,
since it will only work when all source files you intend to check fences
against fall under your rootDirs.
This is the case for OWA, but I don't know what other consumers look
like.

Depends on u/mahuangh/faster-source-providers-2

Quick (completely nonscientific) test for impact on performance:

$ time yarn good-fences --rootDir packages shared -p ./tsconfig.noprojects.json --progressBar -x
Done in 45.17s.

real    0m45.452s
user    0m0.000s
sys     0m0.152s

$ time yarn good-fences --rootDir packages shared -p ./tsconfig.noprojects.json --progressBar
Done in 359.96s.

real    6m0.237s
user    0m0.046s
sys     0m0.121s

[smikula]

This looks great. I wish we didn't have to bend over backwards to get good perf relative to typescript, but the speed gains are impressive! I just left a handful of nitpick comments.