Refactors subsystems to use dependency-ordering to determine init order. Subsystems can now declare their own dependencies. (#90268)

## About The Pull Request
As the title says.
`init_order` is no more, subsystems ordering now depends on their
declared dependencies.
Subsystems can now declare which other subsystems need to init before
them using a list and the subsystem's typepath
I.e.
```dm
dependencies = list(
    /datum/controller/subsystem/atoms,
    /datum/controller/subsystem/mapping
)
```
The reverse can also be done, if a subsystem must initialize after your
own:
```dm
dependents = list(
    /datum/controller/subsystem/atoms
)
```
Cyclical dependencies are not allowed and will throw an error on
initialization if one is found.
There's also a debug tool to visualize the dependency graph, although
it's a bit basic:

![image](https://github.com/user-attachments/assets/80c854d9-c2a5-4f2f-92db-a031e9a8e257)

Subsystem load ordering can still be controlled using `init_stage`, some
subsystems use this in cases where they must initialize first or last
regardless of dependencies. An error will be thrown if a subsystem has
an `init_stage` before one of their dependencies.

## Why It's Good For The Game
Makes dealing with subsystem dependencies easier, and reduces the chance
of making a dependency error when needing to shift around subsystem
inits.

## Changelog
🆑
refactor: Refactored subsystem initialization
/🆑

code/controllers/subsystem/processing/reagents.dm

@@ -2,11 +2,11 @@
 
 PROCESSING_SUBSYSTEM_DEF(reagents)
 	name = "Reagents"
-	init_order = INIT_ORDER_REAGENTS
 	priority = FIRE_PRIORITY_REAGENTS
 	wait = 0.25 SECONDS //You might think that rate_up_lim has to be set to half, but since everything is normalised around seconds_per_tick, it automatically adjusts it to be per second. Magic!
 	flags = SS_KEEP_TIMING
 	runlevels = RUNLEVEL_GAME | RUNLEVEL_POSTGAME
+	init_stage = INITSTAGE_EARLY
 	///What time was it when we last ticked
 	var/previous_world_time = 0