Test FileSystemProvider API

[jrieken]

Refs: #47475

• any @alexandrudima
• any @dbaeumer

Complexity: 3

We have new API that allows extensions to contribute a file system, FTP servers and LiveShare are good examples for this.

This item is about the API mostly, think of a review. Have an implementation for trying out things but don't get stuck in the details of your backend. There is another item that is about implementing a file system provider

Please test and think about the following.

• Is the API using concepts that resonate with you? Does something appear as completely alien?
• Do the comments make sense?
• In some places the API uses booleans instead of enums, e.g to express the file type.
• The API often talks about files which generally means folders, symlinks, and regular files.
• use the MemFs-sample and check if it meets you expectation of how to implement this API

[jrieken]

FYI - to test against the latest API do the following:

• open your extensions package.json file
• change the engine.vscode property to *
• run npm run postinstall (assuming you have the vscode module as dev-dependency)
• undo the engine-version change from step 2

[dbaeumer]

Added my comments to #48527