Add ast-grep rule to prevent String() function usage

Prefer .toString() or .toLocaleString() over String() for
more predictable behavior and consistency.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.ast-grep/rules/no-string-function.yml

@@ -0,0 +1,7 @@
+id: no-string-function
+language: typescript
+severity: error
+message: "Don't use String() - use .toString() or .toLocaleString() instead."
+note: "String() can have unexpected behavior. Use .toString() for general conversion or .toLocaleString() for locale-aware formatting."
+rule:
+  pattern: String($VAL)