Handle nested expressions

src/parser.js

     functionCallNode.args = []
     functionCallNode.subtree = []
 
-    while (!tokenTypesToEndTheLoop.includes(this.tokens.peek().type)) {
+    while (this.tokens.peek().type != tokenTypes.CPAREN) {
       if (this.tokens.peek().type === tokenTypes.ATTRIBUTE) {
         functionCallNode.args.push(this.attribute())
+      } else if (this.tokens.peek().type === tokenTypes.OPAREN) {
+        functionCallNode.subtree = this.expr()
       }
     }