Tests for functionMap.php

[IanDelMar]

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.
• Ensuring the uniqueness of map entries.
• Checking for unmatched parentheses.

[szepeviktor]

Ensuring the uniqueness of map entries.

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