Tests for functionMap.php

As the function map expands, we should incorporate some checks. I suggest:

• [ ] Relocating the parameter name check outside the visitor.
• [ ] Verifying whether types have already been added through other parts of the visitor.
• [x] Ensuring the uniqueness of map entries.
• [ ] Checking for unmatched parentheses.

Ensuring the uniqueness of map entries.

PHPStan does this with every array. https://phpstan.org/r/e722f28d-3c0d-4077-8996-8819813abaad Resolved in #126