Makes gasses use defines (#75542)

## About The Pull Request

Adds defines for gasses and replaces uses I've found to instead use the
defines.

Can you believe I made this PR while trying to work with Xenos? This
sucks!

## Why It's Good For The Game

There's a lot of different uses of things like "o2" and "plasma", and
they are pretty inconsistent. In some places, it's "hydrogen", in others
it's "h2". In some it's "plasma", others "plasm". This unifies it all
under defines so it has a less chance of breaking in the future.

## Changelog

Nothing player-facing.

code/modules/mod/modules/modules_general.dm

@@ -365,7 +365,7 @@
 	/// The temperature we are regulating to.
 	var/temperature_setting = BODYTEMP_NORMAL
 	/// Minimum temperature we can set.
-	var/min_temp = 293.15
+	var/min_temp = T20C
 	/// Maximum temperature we can set.
 	var/max_temp = 318.15