LibJS: Support deleting local variables with operator delete

To make this cleaner i also moved the logic into Reference::delete_.

Userland/Libraries/LibJS/Tests/operators/delete-local-variable.js

@@ -0,0 +1,8 @@
+test("basic functionality", () => {
+    let a = 5;
+    expect(delete a).toBeTrue();
+
+    expect(() => {
+        a;
+    }).toThrowWithMessage(ReferenceError, "'a' is not defined");
+});