Add eslint-plugin-regexp (#24361)

Add
[`eslint-plugin-regexp`](https://github.com/ota-meshi/eslint-plugin-regexp)
and fix discovered issues. Config is mostly the recommended one, but I
relaxed a few rules.

package.json

@@ -65,6 +65,7 @@
     "eslint-plugin-import": "2.27.5",
     "eslint-plugin-jquery": "1.5.1",
     "eslint-plugin-no-jquery": "2.7.0",
+    "eslint-plugin-regexp": "1.14.0",
     "eslint-plugin-sonarjs": "0.19.0",
     "eslint-plugin-unicorn": "46.0.0",
     "eslint-plugin-vue": "9.11.0",