feat: add optimized field load/store instructions for direct method access

Introduce CODE_LOAD_FIELD_THIS and CODE_STORE_FIELD_THIS opcodes that bypass receiver stack operations when field access occurs directly within a method body. Refactor field() compiler to emit these faster instructions when compiler->methodName is set, and extract loadThis() helper to correctly resolve 'this' in nested functions. Update VM interpreter with dedicated dispatch cases that read receiver from frame->stackStart instead of popping, and add debug disassembly support. Add test cases for field closure capture and nested class field isolation, removing related TODO comments.

test/field/nested_class.wren

@@ -0,0 +1,18 @@
+class Outer {
+  method {
+    _field = "outer"
+    IO.write(_field) // expect: outer
+
+    class Inner {
+      method {
+        _field = "inner"
+        IO.write(_field) // expect: inner
+      }
+    }
+
+    (new Inner).method
+    IO.write(_field) // expect: outer
+  }
+}
+
+(new Outer).method