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/rule-tests/__snapshots__/no-string-function-snapshot.yml

@@ -0,0 +1,20 @@
+id: no-string-function
+snapshots:
+  String(123):
+    labels:
+    - source: String(123)
+      style: primary
+      start: 0
+      end: 11
+  String(Date.now()):
+    labels:
+    - source: String(Date.now())
+      style: primary
+      start: 0
+      end: 18
+  String(value):
+    labels:
+    - source: String(value)
+      style: primary
+      start: 0
+      end: 13