Add ESLint rule to enforce Lingui msg usage

Adds a custom ESLint rule 'lingui-msg-rule' that ensures the Lingui _()
function is called with msg`` template literals or plural/select macros,
preventing accidental misuse like _('string') which bypasses i18n.

https://claude.ai/code/session_01JMXXPUgAHiSBGmfGwUojKy

eslint/index.js

@@ -9,6 +9,7 @@ const plugin = {
     'avoid-unwrapped-text': require('./avoid-unwrapped-text'),
     'use-exact-imports': require('./use-exact-imports'),
     'use-prefixed-imports': require('./use-prefixed-imports'),
+    'lingui-msg-rule': require('./lingui-msg-rule'),
   },
 }