feat: make this the implicit receiver for method calls inside class bodies

In the compiler, when a bare name is encountered inside a class definition and it does not resolve to a local variable or global, emit an implicit `this.` load before the call, enabling getter, setter, and method calls without explicit receiver. Update all benchmark, builtin, and test files to remove redundant `this.` qualifiers, and add comprehensive test suites for implicit receiver behavior across instance, inherited, static, nested, and shadowing scenarios.

test/implicit_receiver/instance_methods.wren

@@ -0,0 +1,23 @@
+class Foo {
+  getter {
+    IO.print("getter")
+  }
+
+  setter = value {
+    IO.print("setter")
+  }
+
+  method(a) {
+    IO.print("method")
+  }
+
+  test {
+    getter            // expect: getter
+    setter = "value"  // expect: setter
+    method("arg")     // expect: method
+  }
+}
+
+(new Foo).test
+
+// TODO: Need to decide how these interact with globals.