Run Playwright on all platforms in PRs (#7304)

# Description of Changes
Nightlies keep failing because the Playwright tests only run on Chrome
in PRs. This PR changes it so that we run all 3 browsers in all
(frontend) PRs so we catch these things before they merge in. They run
in parallel so it won't take any more time for the CI to finish.

.taskfiles/e2e.yml

@@ -15,6 +15,15 @@ tasks:
     cmds:
       - npx playwright test --project=stubbed {{.CLI_ARGS}}
 
+  stubbed-project:
+    desc: "Run the stubbed E2E suite for a single Playwright project"
+    dir: frontend/editor
+    deps: [ ':frontend:prepare' ]
+    vars:
+      PROJECT: '{{.PROJECT | default "stubbed"}}'
+    cmds:
+      - npx playwright test --project={{.PROJECT}} {{.CLI_ARGS}}
+
   live:
     desc: "Run live E2E tests"
     summary: |