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.

test/e2e/models/CodeServer.ts

@@ -88,6 +88,8 @@ export class CodeServer {
           path.join(dir, "config.yaml"),
           "--user-data-dir",
           dir,
+          "--extensions-dir",
+          path.join(__dirname, "../extensions"),
           // The last argument is the workspace to open.
           dir,
         ],