Allow localhosts to set "dev override preferences" to load a specific preference savefile for guests (#92770)

## About The Pull Request

A verb is now available on localhost called `"Export Save as Dev
Preferences"`

This exports your current savefile to `/config/dev_preferences.json`

If you then connect to your localhost as a guest, it will load
`dev_preferences.json` as your preference datum

This allows for devs testing the game locally to load preferences for
guests.
(Guests connecting to live servers are completely unaffected.)

## Why It's Good For The Game

Initially I only did this because the recent keybinding changes have
destroyed my muscle memory when testing w/o logging in.

But as I worked on it I thought of a few other usecases, like when
implementing preference version migration - the dev preference is never
saved which means you can re-compile as much as you want without needing
to revert your save manually.

code/datums/json_savefile.dm

@@ -118,3 +118,7 @@ GENERAL_PROTECT_DATUM(/datum/json_savefile)
 		return TRUE
 
 	return FALSE
+
+/// Copies the entire tree to another json savefile datum, overwriting whatever was in the other datum before.
+/datum/json_savefile/proc/copy_to_savefile(datum/json_savefile/other_savefile)
+	other_savefile.tree = tree.Copy()