test and rework: Added push test and cahnged arena_push return

So added a little test that just adds 3 ints to an arena, nothing much
but the basics work, also changed arena_push because it only pushed and
didn't returned a ponter (wich is fucking useless) so also fixed that,
everything seems fine now, need more tests.

# Tipos:
# feat, fix, refactor, docs, style, test, chore

include/arena.h

@@ -50,7 +50,7 @@ ArenaResult arena_init(size_t capacity);
 void arena_destroy(Arena *arena);
 
 ArenaPointer arena_alloc(Arena *arena, size_t size, size_t alignment);
-ArenaErr arena_push(Arena *arena, void *data, size_t size, size_t alignment);
+ArenaPointer arena_push(Arena *arena, void *data, size_t size, size_t alignment);
 ArenaErr arena_realloc(Arena *arena, size_t new_capacity);
 
 SizeResult align_arena_offset(Arena *arena, size_t alignment);