pub fn parser(mut input: ItemFn) -> TokenStream {
    if let Some(abi) = &mut input.sig.abi {
        abi.name = Some(LitStr::new(ABI, Span::call_site()));
    }
    input.to_token_stream()
}
Parses a function and sets the correct ABI to interact with PHP depending
on the OS.

On Windows, this sets the extern ABI to vectorcall while on all other OS
it sets it to C.
crates/macros/src/fastcall.rs