feat: add static method support to class definitions with metaclass dispatch

Add TOKEN_STATIC keyword to the lexer, replacing the unused TOKEN_META, and extend the method() compiler function with an isStatic parameter. When isStatic is true, emit a new CODE_METACLASS instruction before CODE_METHOD to push the class's metaclass onto the stack, enabling static method dispatch. Also fix a buffer overflow bug in string_plus by allocating one extra byte for the null terminator, and refactor object initialization across vm.c by introducing a shared initObj helper to eliminate repeated type/flag assignments.

test/method_static.wren

@@ -0,0 +1,13 @@
+class Foo {
+  bar { "on instance" }
+  static bar { "on metaclass" }
+
+  bar(arg) { "on instance " + arg }
+  static bar(arg) { "on metaclass " + arg }
+}
+
+io.write("on metaclass " + "arg") // expect: on metaclass arg
+io.write(Foo.new.bar)        // expect: on instance
+io.write(Foo.bar)            // expect: on metaclass
+io.write(Foo.new.bar("arg")) // expect: on instance arg
+io.write(Foo.bar("arg"))     // expect: on metaclass arg