Check_keybindings now only saves keybinds rather than everything (#5120)

# About the pull request

This PR is a followup to #5110 and #2926 making it so we only save the
`key_bindings` value rather than all preferences. I'd be inclined to
just not save at all, but the initial commit on the ported input system
has us saving `remembered_key_bindings` just a few lines down anyways so
no matter what we're writing something.

# Explain why it's good for the game

Should better future proof this trampling preferences, but still retain
existing behavior where any automatically rebound keys are immediately
saved.

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl: Drathek
code: Reduced most preference re-saving when preferences are initially
loaded
/:cl:

code/modules/client/preferences_savefile.dm

@@ -274,6 +274,7 @@
 	vars["fps"] = fps
 
 	check_keybindings()
+	S["key_bindings"] << key_bindings
 
 	if(remembered_key_bindings)
 		for(var/i in GLOB.keybindings_by_name)
@@ -652,7 +653,7 @@
 					addedbind = TRUE
 		if(!addedbind)
 			notadded += kb
-	save_preferences()
+
 	if(length(notadded))
 		addtimer(CALLBACK(src, PROC_REF(announce_conflict), notadded), 5 SECONDS)