Adds an universal object health system. (#21266)

https://www.youtube.com/watch?v=ZpUYjpKg9KY

As per title. The goal is to add health to most objects in the game and
thus allow them to be dynamically destructible, while also avoiding
shitcode like each structure having its own snowflake health update
proc.

---------

Signed-off-by: Matt Atlas <mattiathebest2000@hotmail.it>
Co-authored-by: Matt Atlas <liermattia@gmail.com>
Co-authored-by: SleepyGemmy <99297919+SleepyGemmy@users.noreply.github.com>

code/modules/shieldgen/emergency_shield.dm

@@ -7,7 +7,7 @@
 	opacity = FALSE
 	anchored = FALSE
 	req_access = list(ACCESS_ENGINE)
-	var/health = 100
+	maxhealth = 100
 	var/active = FALSE
 	/// Malfunction causes parts of the shield to slowly dissipate
 	var/malfunction = FALSE
@@ -270,8 +270,7 @@
 	anchored = TRUE
 	unacidable = TRUE
 	atmos_canpass = CANPASS_NEVER
-	/// The shield can only take so much beating (prevents perma-prisons)
-	var/health = 75
+	maxhealth = 75
 	/// How much power we use when first creating the shield
 	var/shield_generate_power = 75 KILO WATTS
 	/// How much power we use when just being sustained.