Extend handler flags to have `handler_first` and `handler_last` for registered callback and similar that uses the handler flags.

[knryuu]

As discussed on Discord, IS4 said about extending handler flags to have handler_first and handler_last for the "hook order", since the current order depends on you calling the function.

For example:

pawn_register_callback("OnPlayerConnect", "FirstCall");
pawn_register_callback("OnPlayerConnect", "SecondCall");
// ...

Extending the flag to support explicit calling will make sure the function works as the developer intended.
Supposed you want to hook the OnPlayerConnect callback but you want that to be called at the very last, with the new flag you can just do this:

pawn_reigster_callback("OnPlayerConnect", "ResetVariable", handler_last);

This will ensure the ResetFunction gets called last, no matter where you place the function register calls.

The same goes with handler_first:

pawn_reigster_callback("OnPlayerConnect", "PrepareVariable", handler_first);

This function will make sure the PrepareVariable gets called first, no matter where you place the function register calls.