fix(@schematics/angular): improve VS Code background compilation start/end detection

The task is initially "active", and should be considered "completed" when the bundle generation
result is displayed, even if the generation failed. The task should only switch back to "active"
when changes are detected, not on any arbitrary output by `ng watch`.

This change enables proper handling of re-attaching the debugger while the background task is
already running.

Fixes #32111

Author: karcherm

packages/schematics/angular/workspace/files/__dot__vscode/tasks.json.template

@@ -12,10 +12,10 @@
         "background": {
           "activeOnStart": true,
           "beginsPattern": {
-            "regexp": "(.*?)"
+            "regexp": "Changes detected"
           },
           "endsPattern": {
-            "regexp": "bundle generation complete"
+            "regexp": "bundle generation (complete|failed)"
           }
         }
       }
@@ -30,10 +30,10 @@
         "background": {
           "activeOnStart": true,
           "beginsPattern": {
-            "regexp": "(.*?)"
+            "regexp": "Changes detected"
           },
           "endsPattern": {
-            "regexp": "bundle generation complete"
+            "regexp": "bundle generation (complete|failed)"
           }
         }
       }