Tasks:
- Extract common `undefined | null` logic everywhere to reuse
(now-exported) type `Nullish`
- Expose `FileMatcher` by removing `@internal` flag (why was it internal
to begin with?)
- Created base type `ObjectMap` to reduce writing `{ [key: string]:
<anything> }` everywhere.
- Updated eslint to register `test/tsconfig.json`
- Fixes `skipLibCheck: false` by forcing UUID.ts to return a `Buffer`
instead of `Buffer<ArrayBuffer>`, which the compiler doesn't like
depending on version of node you're on

)