chore(ci): Report diff in _diff:fail in GitHub Actions

package.json

@@ -19,7 +19,7 @@
     "_check:registry": "npx gulp validate-registry",
     "_check:text": "npx textlint content data `ls *.md`",
     "_diff:check": "git diff --name-only --exit-code",
-    "_diff:fail": "git diff --name-only --exit-code || exit 1",
+    "_diff:fail": "if ! git diff --name-only --exit-code; then if [ -n \"$GITHUB_STEP_SUMMARY\" ]; then ( echo '## git diff'; echo '```diff'; echo; git diff; echo '```' ) >> \"$GITHUB_STEP_SUMMARY\"; fi; exit 1; fi",
     "_fail": "exit 1",
     "_filename-error": "echo 'ERROR: the following files violate naming conventions; fix using: `npm run fix:filenames`'; echo; npm run -s _ls-bad-filenames; exit 1",
     "_get:no": "echo SKIPPING get operation",