feat: proxy URI environment variable (#4681)

* Add test extension

This will let us test extension-related features (like the proxy URI).

I removed the environment variables in the script because they override
the ones you set yourself. We still set defaults in constants.ts.

* Add changelog entry for VSCODE_PROXY_URI

* Add terminal test for VSCODE_PROXY_URI

* Update VS Code

This adds the VSCODE_PROXY_URI environment variable.

CHANGELOG.md

@@ -40,6 +40,10 @@ implementation (#4414).
 - The last opened folder/workspace is no longer stored separately in the
   settings file (we rely on the already-existing query object instead).
 
+### Added
+
+- `VSCODE_PROXY_URI` env var for use in the terminal and extensions.
+
 ### Deprecated
 
 - `--link` is now deprecated (#4562).