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_setter.wren

@@ -0,0 +1,20 @@
+class Foo {
+  bar = value {
+    IO.print("setter")
+    return value
+  }
+
+  test {
+    bar = "value" // expect: setter
+
+    {
+      bar = "value" // expect: setter
+      var bar = "local"
+      bar = "value" // no expectation
+    }
+
+    bar = "value" // expect: setter
+  }
+}
+
+(new Foo).test