Add code to migrate old invalid NIF paths in save files (#3615)

## About The Pull Request

`modular_persistence.json` on player saves holds the path of any NIFs
they have installed and ended the round with, but when we refactored
`/obj/item/organ/internal` to just be `/obj/item/organ` these save files
weren't updated, meaning people lost their NIFs and now we get runtime
errors when an affected person joins the game. This code will check
their save on load and update the path appropriately (which will then be
written to the file at round end)

## Why It's Good For The Game

They lost NIFs due to a bug

## Proof Of Testing

I can't really do pictures for this but I got a save file that had
invalid NIFs installed and used it to test locally and it worked, also
tested save files without NIFs and clients that don't have a
`modular_persistence.json` at all, no runtimes

## Changelog
🆑
fix: characters who had their NIFs disappear a few months ago should
have them back now
/🆑

modular_skyrat/modules/modular_implants/code/nif_persistence.dm

@@ -75,6 +75,10 @@
 	if(!persistence.nif_path)
 		return
 
+	// if nif_path exists but isn't a valid path it probably needs to be migrated post-organ refactor
+	if(!text2path(persistence.nif_path))
+		persistence.nif_path = replacetext(persistence.nif_path, "/obj/item/organ/internal", "/obj/item/organ")
+
 	var/obj/item/organ/cyberimp/brain/nif/new_nif = new persistence.nif_path
 
 	new_nif.durability = persistence.nif_durability