fix(@angular-devkit/build-angular): correctly generate ServiceWorker config on Windows

Since #20518, the generation of the ServiceWorker configuration has been
broken on Windows. The reason is the use of `path.posix.*` methods on
non-POSIX paths, resulting in broken paths. I.e. we ended up with
something like the following:

```js
path.posix.relative('C:\\foo', 'C:\\foo\\bar/baz');
// Expected result: `bar/baz`
// Actual result:   `../C:\\foo\\bar/baz`
```

This caused the config generator to fail to find any files and thus fail
to populate the config with cacheable assets.

This commit fixes this by using platform-specific `path.*` methods for
path manipulation and manually normalizing the path separators before
returning the results.

Fixes #20894

Author: gkalpak

packages/angular_devkit/build_angular/src/utils/service-worker.ts

@@ -46,7 +46,7 @@ class CliFilesystem implements Filesystem {
     for await (const entry of await fs.opendir(dir)) {
       if (entry.isFile()) {
         // Uses posix paths since the service worker expects URLs
-        items.push('/' + path.posix.relative(this.base, path.posix.join(dir, entry.name)));
+        items.push('/' + path.relative(this.base, path.join(dir, entry.name)).replace(/\\/g, '/'));
       } else if (entry.isDirectory()) {
         subdirectories.push(path.join(dir, entry.name));
       }