Fixes stabilized red bypassing immutable slowdowns / Fixes being unable to construct fulltile objects on immutably slow things (#82250)

## About The Pull Request

- Fixes Stabilized Red extract's equipment slowdown immunity bypassing
item Immutable Slowdown

- Fixes(?) Settler equipment slowdown modifier applying to immutable
slows

- Fixes Immutable Slow being considered an object flag when it was an
item flag, causing objects to consider objects with it to be
`BLOCKS_CONSTRUCTION_DIR`

## Why It's Good For The Game

The description of Immutable Slows: 

`When players should not be able to change the slowdown of the item
(Speed potions, etc)`

Stabilized Red extracts were changing the slowdown of the item, which is
unintended.
Likewise Settler was doing the same, but that one I'm a bit more iffy
on.

Either way I suppose if things should immutably be slow, they should
immutably be slow.

## Changelog

🆑 Melbert
fix: Stabilized Red extracts no longer bypass Immutably Slow items
fix: Settler equipment speed modifier no longer applied to Immutably
Slow items
fix: Immutably Slow items no longer block construction of certain items 
/🆑

code/modules/movespeed/modifiers/mobs.dm

@@ -30,10 +30,14 @@
 	movetypes = FLYING
 	variable = TRUE
 
+/// Movespeed modifier applied by worn equipment.
 /datum/movespeed_modifier/equipment_speedmod
 	variable = TRUE
 	blacklisted_movetypes = FLOATING
 
+/// Movespeed modifier applied by immutably slow worn equipment. Should never be ignored, because that's the point.
+/datum/movespeed_modifier/equipment_speedmod/immutable
+
 /datum/movespeed_modifier/grab_slowdown
 	id = MOVESPEED_ID_MOB_GRAB_STATE
 	blacklisted_movetypes = FLOATING