Add maint script: find_unused_settings.py. NFC (#24869)

Does what it says on the tin. I wrote this suspecting we might have some
but it turns out we don't. Adding the script anyway as it could be
useful in future and as the basis for other such scripts.

Author: sbc100

tools/maint/find_unused_settings.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+"""Find unused settings from settings.js and settings_internal.js.
+"""
+
+import os
+import subprocess
+import sys
+
+script_dir = os.path.dirname(os.path.abspath(__file__))
+root_dir = os.path.dirname(os.path.dirname(script_dir))
+
+sys.path.insert(0, root_dir)
+
+# This avoiding including `LEGACY_SETTINGS`
+os.environ['EMCC_STRICT'] = '1'
+
+from tools.settings import settings
+
+
+def main():
+  print(f'Searching {len(settings.attrs)} settings')
+  for key in settings.attrs:
+    cmd = ['git', 'grep', '-q', f'\\<{key}\\>',  ':(exclude)src/settings.js', ':(exclude)src/settings_internal.js']
+    print('CHECKING:', key)
+    # git grep returns 0 if there is a match and non-zero when there is not
+    if subprocess.run(cmd, check=False).returncode:
+      print('NOT FOUND: ', key)
+
+
+if __name__ == '__main__':
+  sys.exit(main())