[Port] Moves base_icon_state to base of atom and adds it to mechs

[PestoVerde322]

About The Pull Request

This is a port of https://github.com/Monkestation/MonkeStation/pull/277

Porting: "Moves base_icon_state to the base of atom" https://github.com/tgstation/tgstation/pull/52917 "mechs now use base_icon_state for their icon changes" https://github.com/tgstation/tgstation/pull/56480

Atomizes base_icon_state since multiple objects across the game use it*. Then adds a base_icon_state instead of initial(icon_state) to mechs, which "allows custom icons without the mech disappearing, fun!"

*worth mentioning that there may be more objects that have a variable like this under different names.

Why It's Good For The Game

More modern code, good for coders.

Testing Photographs and Procedure

Screenshots&Videos

The game runs and there's no real way to proof that anything changed it aside an in-game screen, so have one:

Changelog

:cl: refactor: base_icon_state is now an atomized variable code: mechs now use base_icon_state for their icon changes /:cl: