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/no-string-function-test.yml

@@ -0,0 +1,13 @@
+id: no-string-function
+valid:
+  # toString() is fine
+  - value.toString()
+  - (123).toString()
+  - date.toLocaleString()
+  # Other functions named String are fine
+  - myString(value)
+invalid:
+  # String() function should be flagged
+  - String(value)
+  - String(123)
+  - String(Date.now())