Add Reassure mobile performance testing library

.github/workflows/performance.yml

@@ -0,0 +1,7 @@
+- name: Run mobile performance testing script
+  run: ./gutenberg/bin/reassure-tests.sh
+
+- name: Report mobile performance results
+  run: npm danger ci
+  env:
+    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

dangerfile.ts

@@ -0,0 +1,6 @@
+import path from 'path';
+import { dangerReassure } from 'reassure';
+
+dangerReassure( {
+	inputFilePath: path.join( __dirname, 'gutenberg/.reassure/output.md' ),
+} );