Replaced 'set' with direct assignment where applicable (item db)

This also includes various other tweaks to the item db scripts,
including the newly introduced min() and max() commands.

Signed-off-by: Haru <haru@dotalux.com>
Marked 'menu' script command as deprecated.

Note: Scripts in npc/custom haven't been updated to remove usage of
said script commands yet.

Signed-off-by: Haru <haru@dotalux.com>