Initialize environment service with isBuilt property in standalone theme service (#278734)

* Initial plan

* Initialize _environment with isBuilt: false for standalone editor

Co-authored-by: alexr00 <[email protected]>

* Simplify _environment initialization to use object literal

Co-authored-by: alexr00 <[email protected]>

Author: Copilot

src/vs/editor/standalone/browser/standaloneThemeService.ts

@@ -222,7 +222,7 @@ export class StandaloneThemeService extends Disposable implements IStandaloneThe
 	private readonly _onProductIconThemeChange = this._register(new Emitter<IProductIconTheme>());
 	public readonly onDidProductIconThemeChange = this._onProductIconThemeChange.event;
 
-	private readonly _environment: IEnvironmentService = Object.create(null);
+	private readonly _environment: IEnvironmentService = { isBuilt: false } as IEnvironmentService;
 	private readonly _knownThemes: Map<string, StandaloneTheme>;
 	private _autoDetectHighContrast: boolean;
 	private _codiconCSS: string;