Standardize the extension installation path to `/share/vscode/extensions`
regardless of the package name.
Previously, setting the package name to 'antigravity' would change the
internal path to `/share/antigravity`, which caused a mismatch because
most upstream Nixpkgs extensions are hardcoded to install into
`/share/vscode/extensions`. This resulted in an empty or incomplete
extensions directory in the nix store.
Also added 'antigravity' to the list of packages requiring an
immutable extensions.json file.
Signed-off-by: unlsycn <unlsycn@unlsycn.com>
b593765639