feat(compiler): Namespace CSS variables to the app (#68846)

Adds logic to inject symbols into CSS variables for runtime namespacing.
The runtime now replaces instances of `%NS%` with a namespacing
variable, limiting reach of CSS variables to the current app. An opt-out
syntax of a `--global` prefix allows users to avoid this behavior.

PR Close #68846

goldens/public-api/platform-browser/index.api.md

@@ -54,6 +54,15 @@ export class By {
 // @public
 export function createApplication(options?: ApplicationConfig, context?: BootstrapContext): Promise<ApplicationRef>;
 
+// @public
+export class CssVarNamespacer {
+    namespace(name: string): string;
+    // (undocumented)
+    static ɵfac: i0.ɵɵFactoryDeclaration<CssVarNamespacer, never>;
+    // (undocumented)
+    static ɵprov: i0.ɵɵInjectableDeclaration<CssVarNamespacer>;
+}
+
 // @public
 export function disableDebugTools(): void;
 
@@ -158,6 +167,9 @@ export const platformBrowser: (extraProviders?: StaticProvider[]) => PlatformRef
 // @public
 export function provideClientHydration(...features: HydrationFeature<HydrationFeatureKind>[]): EnvironmentProviders;
 
+// @public
+export function provideCssVarNamespacing(namespace: string): EnvironmentProviders;
+
 // @public
 export function provideProtractorTestingSupport(options?: {
     usePendingTasksForStability?: boolean;