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/this/nested_class.wren

@@ -1,13 +1,13 @@
 class Outer {
   method {
-    IO.print(this.toString) // expect: Outer
+    IO.print(this) // expect: Outer
 
     fn {
-      IO.print(this.toString) // expect: Outer
+      IO.print(this) // expect: Outer
 
       class Inner {
         method {
-          IO.print(this.toString) // expect: Inner
+          IO.print(this) // expect: Inner
         }
         toString { return "Inner" }
       }