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/closure.wren

@@ -0,0 +1,16 @@
+class Foo {
+  new { _field = "Foo field" }
+
+  closeOverGet {
+    return fn { return _field }
+  }
+
+  closeOverSet {
+    return fn { _field = "new value" }
+  }
+}
+
+var foo = new Foo
+IO.write(foo.closeOverGet.call) // expect: Foo field
+foo.closeOverSet.call
+IO.write(foo.closeOverGet.call) // expect: new value