Merge multiple "holding" tags

BUR #192 has been rejected.

create alias holding_gun -> holding_weapon
create alias holding_sword -> holding_weapon
create alias holding_axe -> holding_weapon
create alias holding_scythe -> holding_weapon

4 tags combined into 1 since gun, sword, axe, and scythe are all weapons.

I dont actually agree with this one. Rather holding weapon should be tagged along with all of those, but maybe some people just want to find a picture with a gun, not a sword, so they are distinct.

The bulk update request #192 (forum #16433) has been rejected by @yomihara-exec.