feat: implement class inheritance with 'is' keyword and superclass method copying

Add support for class inheritance in the compiler by parsing the 'is' keyword to load a superclass and emit a CODE_SUBCLASS instruction. In the VM, extend newClass to accept a superclass parameter, copy inherited methods from the superclass into the new class, and store the superclass pointer in the ObjClass struct. Introduce a root Object class in the core library and set it as the superclass for all built-in types. Add a test file verifying method inheritance and arity-based dispatch.

test/class_inheritance.wren

@@ -0,0 +1,26 @@
+class Foo {
+  methodOnFoo { io.write("foo") }
+  method(a) { io.write("foo") }
+  method(a, b, c) { io.write("foo") }
+}
+
+class Bar is Foo {
+  methodOnBar { io.write("bar") }
+  method(a, b) { io.write("bar") }
+  method(a, b, c, d) { io.write("bar") }
+}
+
+var bar = Bar.new
+bar.methodOnFoo // expect: foo
+bar.methodOnBar // expect: bar
+
+// Methods with different arity do not shadow each other.
+bar.method(1) // expect: foo
+bar.method(1, 2) // expect: bar
+bar.method(1, 2, 3) // expect: foo
+bar.method(1, 2, 3, 4) // expect: bar
+
+// TODO(bob): Overriding (or BETA-style refining?).
+// TODO(bob): Private fields.
+// TODO(bob): Super (or inner) calls.
+// TODO(bob): Grammar for what expressions can follow "is".