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/locals_shadow_getter.wren

@@ -0,0 +1,17 @@
+class Foo {
+  bar { return "getter" }
+
+  test {
+    IO.print(bar) // expect: getter
+
+    {
+      IO.print(bar) // expect: getter
+      var bar = "local"
+      IO.print(bar) // expect: local
+    }
+
+    IO.print(bar) // expect: getter
+  }
+}
+
+(new Foo).test