feat: add foreign class support with allocation, construct opcodes, and CLI callback wiring

Implement the initial infrastructure for foreign classes in the Wren VM, including new opcodes (FOREIGN_CONSTRUCT, FOREIGN_CLASS), a WrenBindForeignClassFn callback type in the public API, and updated CLI vm.c to store and forward foreign binding callbacks via setForeignCallbacks(). The compiler now tracks whether a class is foreign via a new isForeign flag in ClassCompiler, emits CODE_FOREIGN_CONSTRUCT instead of CODE_CONSTRUCT for foreign class constructors, and errors on field definitions inside foreign classes. The debug dump and opcode header gain entries for the new opcodes, and wrenBindSuperclass handles the -1 numFields sentinel for foreign classes to prevent field inheritance from superclasses.

test/language/class/field_in_foreign_class.wren

@@ -0,0 +1,9 @@
+foreign class Foo {
+  bar {
+    // Can't read a field.
+    IO.print(_bar) // expect error
+
+    // Or write one.
+    _bar = "value" // expect error
+  }
+}

test/language/class/foreign_class_inherit_fields.wren

@@ -0,0 +1,7 @@
+class Foo {
+  method() {
+    _field = "value"
+  }
+}
+
+foreign class Bar is Foo {} // expect runtime error: Foreign class 'Bar' may not inherit from a class with fields.

test/language/class/missing_class_after_foreign.wren

@@ -0,0 +1,2 @@
+// Missing "class".
+foreign blah A {} // expect error