fix(@schematics/angular): add **/__screenshots__ to .gitignore

Vitest Browser mode generates screenshots on failed tests by default.

Author: cexbrayat

packages/schematics/angular/workspace/files/__dot__gitignore.template

@@ -36,6 +36,7 @@ yarn-error.log
 /libpeerconnection.log
 testem.log
 /typings
+**/__screenshots__
 
 # System files
 .DS_Store