Enable a few more eslint-plugin-regexp rules

The remaining rules from the plugin's "Best Practices" category that the
recommended config leaves off, plus the two it only warns about, all of
which the code already complies with.

`regexp/prefer-regexp-test` is left off, since `unicorn/prefer-regexp-test`
already covers it.

eslint.config.mjs

@@ -62,7 +62,13 @@ export default [
     rules: {
       ...regexpPlugin.configs["flat/recommended"].rules,
       "regexp/no-legacy-features": "off",
+      "regexp/no-octal": "error",
+      "regexp/no-potentially-useless-backreference": "error",
+      "regexp/no-standalone-backslash": "error",
       "regexp/no-super-linear-move": "error",
+      "regexp/optimal-lookaround-quantifier": "error",
+      "regexp/prefer-escape-replacement-dollar-char": "error",
+      "regexp/prefer-regexp-exec": "error",
     },
   },
   {