/tg/ pref datums (part 1) (#16219)

* TG Prefs (Step 1: JSON savefiles)

* TG Prefs (Step 2: Preference Datum Code)

* TG Prefs (Step 3: Convert /datum/client_preferences)

* TG Prefs (Step 4: Clean up and finishing touches)

* Fix some weird compile errors from the rebase

code/datums/browser.dm

@@ -27,7 +27,7 @@
 	if (nref)
 		ref = nref
 	// If a client exists, but they have disabled fancy windowing, disable it!
-	if(user && user.client && !user.client.is_preference_enabled(/datum/client_preference/browser_style))
+	if(!user?.client?.prefs?.read_preference(/datum/preference/toggle/browser_style))
 		return
 	add_stylesheet("common", 'html/browser/common.css') // this CSS sheet is common to all UIs