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_closure.wren

@@ -3,7 +3,7 @@ class Foo {
     return fn {
       return fn {
         return fn {
-          return this.toString
+          return toString
         }
       }
     }