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/nested_class.wren

@@ -0,0 +1,47 @@
+class Outer {
+  getter {
+    IO.print("outer getter")
+  }
+
+  setter = value {
+    IO.print("outer setter")
+  }
+
+  method(a) {
+    IO.print("outer method")
+  }
+
+  test {
+    getter            // expect: outer getter
+    setter = "value"  // expect: outer setter
+    method("arg")     // expect: outer method
+
+    class Inner {
+      getter {
+        IO.print("inner getter")
+      }
+
+      setter = value {
+        IO.print("inner setter")
+      }
+
+      method(a) {
+        IO.print("inner method")
+      }
+
+      test {
+        getter            // expect: inner getter
+        setter = "value"  // expect: inner setter
+        method("arg")     // expect: inner method
+      }
+    }
+
+    (new Inner).test
+
+    getter            // expect: outer getter
+    setter = "value"  // expect: outer setter
+    method("arg")     // expect: outer method
+  }
+}
+
+(new Outer).test