8951: internal: migrate to vscode.FileSystem API r=matklad a=wxb1ank
I encountered an error where `bootstrap()` attempts to create a directory with the path `C:\C:\...`. I couldn't find this reported anywhere else. Using the `vscode.FileSystem` API instead of the `fs` one works here. I assume the latter automatically prepends `C:\` to paths whereas the former does not. I don't know if this suggests `vscode.FileSystem` should be used in more places for consistency.
Co-authored-by: wxb1ank <wxblank@gmail.com>