feat: add Meta class with eval method and WREN_USE_LIB_META flag

Introduce a new Meta built-in class exposing a static eval(_) method for runtime code interpretation, guarded by the WREN_USE_LIB_META compile-time flag (default on). Include the meta.wren source, C implementation files, header, VM integration in wrenNewVM, test runner update to include the meta test directory, and an initial test for evaluating code that modifies an existing scoped variable.

builtin/meta.wren

@@ -0,0 +1 @@
+class Meta {}