Improves fslib

[arcanis]

What's the problem this PR addresses?

• The fslib didn't account for some families of functions (appendFile, readFile, writeFile) being allowed to consume file descriptors.

• It also didn't implement appendFile at all.

How did you fix it?

I've added a new type, FSPath<T>, which encompasses T | number. This type is then used as input for the specific functions referenced above. Since most fake fs layers are abstracted under the ProxiedFS abstract, most of them don't have to deal with this as the fds will just be forwarded untransformed.

Of note: TypeScript doesn't prevent a subclass from implementing a subset of an abstract method, meaning that it'll not report it if a subclass of FakeFS<T> uses T as parameter for the relevant functions rather than FSPath<T>. It'll crash at runtime, however, as they'll still receive integers.

Which packages would need a new release (if any)?

berry-fslib, berry-pnp, berry-core, berry-cli

Have you run yarn version [major|minor|patch] --deferred in those packages?

• Yes