fix(@angular-devkit/build-angular): implement relative method in extractor FS

This will be needed when the changes in angular/angular#39006 is merged.

Author: alan-agius4

packages/angular_devkit/build_angular/src/extract-i18n/ivy-extract-loader.ts

@@ -51,7 +51,7 @@ export default function localizeExtractLoader(
   }
 
   // Setup a virtual file system instance for the extractor
-  // * MessageExtractor itself uses readFile and resolve
+  // * MessageExtractor itself uses readFile, relative and resolve
   // * Internal SourceFileLoader (sourcemap support) uses dirname, exists, readFile, and resolve
   const filesystem = {
     readFile(path: string): string {
@@ -63,6 +63,9 @@ export default function localizeExtractLoader(
         throw new Error('Unknown file requested: ' + path);
       }
     },
+    relative(from: string, to: string): string {
+      return nodePath.relative(from, to);
+    },
     resolve(...paths: string[]): string {
       return nodePath.resolve(...paths);
     },