LibJS: Allow "delete someGlobalVariable"

This is solved by allowing Identifier nodes to produce a Reference with
the global object as base.

Libraries/LibJS/Tests/delete-global-variable.js

@@ -0,0 +1,16 @@
+load("test-common.js");
+
+try {
+    a = 1;
+    assert(delete a === true);
+
+    assertThrowsError(() => {
+        a;
+    }, {
+        error: ReferenceError
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}