hook: move is_known_hook() to hook.c for wider use

hook: move is_known_hook() to hook.c for wider use

Move is_known_hook() from builtin/hook.c (static) into hook.c and
export it via hook.h so it can be reused.

Make it return bool and the iterator `h` for clarity (iterate hooks).

Both meson.build and the Makefile are updated to reflect that the
header is now used by libgit, not the builtin sources.

The next commit will use this to reject hook friendly-names that
collide with known event names.

Co-authored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>