Safe mode: search for extends_statement node instead of accessing it by index

[fstxz]

Fixes #173

When class_name_statement is annotated and extends_statement is on the same line as class_name_statement, then extends_statement node is the third child, not second as was described in a comment. This would prevent GdTree::move_extends_statement method from finding the extends_statement node and moving it.

This PR adds a function that searches for a child node with a specific grammar name instead of accessing the node by index.

[NathanLovato]

Thank you very much!