Replace the implementation that relied on symlinks with the one that
create regular files that act like symlinks: they contain a reference to
the local directory where to find the extension.
- Extensions on Windows now enabled through the `sh.exe` interpreter
- `sh.exe` now found on Windows when git was installed via scoop
- `gh extensions list` command shows origin repo for the extension
- `gh extensions upgrade --all` is required to upgrade all extensions
- Added `gh extensions remove`
- Shell completions now include aliases and extension names
- `gh` help output now lists available extension names
- Extensions are stored to XDG_DATA_HOME
