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.

builtin/core.wren

@@ -1,7 +1,7 @@
 class List {
   toString {
     var result = "["
-    for (i in 0...this.count) {
+    for (i in 0...count) {
       if (i > 0) result = result + ", "
       result = result + this[i].toString
     }