LibJS: Implement modulo assignment operator (%=)

Author: https://github.com/linusg
Commit: 72d2bd56ce
Pull-request: https://github.com/SerenityOS/serenity/pull/2105

Libraries/LibJS/Tests/assignment-operators.js

@@ -23,6 +23,10 @@ try {
     assert((x /= 2) === 3);
     assert(x === 3);
 
+    x = 6;
+    assert((x %= 4) === 2);
+    assert(x === 2);
+
     x = 2;
     assert((x **= 3) === 8);
     assert(x === 8);