Returns if the weapon is in silenced mode.
Only the USP and M4A1 can return 1 as they are the only guns in the
game that have a silenced fire mode.
This native does not check that the provided entity is actually a
weapon entity. It will return incorrect values for non-weapon entities.
1 if the weapon is in silenced mode, 0 otherwise
If an invalid entity index or a client index is provided,
an error will be thrown.