Adds a createFolderMount util

[arcanis]

Note

This PR is stacked on top of #6161

What's the problem this PR addresses?

Creating a folder mount requires a bit of non-trivial boilerplate. Not that much code, but a little tricky to remember.

How did you fix it?

Added two functions:

• applyFsLayer applies a FakeFS<PortablePath> on top of the fs module.
• MountFS.createFolderMount returns a MountFS that replaces all access to folder X by accesses to folder Y.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

Bad branch, reopening