Implement FileSystemProvider

[jrieken]

Refs: #47475

• macOS @bpasero

Complexity: 4

This items is about implementing a real file system provider such that we can validate our file system provider API and find gaps. The outcome should be a prototype of a file-FileSystemProvider, e.g. move the logic from the FileService into a FileSystemProvider and see what's not possible, e.g permissions.

[bpasero]

I moved the result of my file system provider to https://github.com/Microsoft/vscode-extension-samples/tree/master/nodefs-provider-sample