Add a DISABLE_DREAMLUAU define, for disabling dreamluau at compile-time (#89359)

## About The Pull Request

this adds a new define, `DISABLE_DREAMLUAU` (commented out by default),
which does... exactly what it says on the tin. it fully disables any
dreamluau-related code (the "Open Lua Editor" admin verb is left in,
albeit just giving the user a warning saying Lua is disabled, just so
there's no confusion about the verb itself being missing)

when compiling with OpenDream outside of CI (so dreamluau code will
still be linted), `DISABLE_DREAMLUAU` will be defined by default, tho.

## Why It's Good For The Game

makes OpenDream testing easier

## Changelog

no user-facing changes. or even code changes for most cases.

code/controllers/subsystem/lua.dm

@@ -1,3 +1,4 @@
+#ifndef DISABLE_DREAMLUAU
 SUBSYSTEM_DEF(lua)
 	name = "Lua Scripting"
 	runlevels = RUNLEVEL_LOBBY | RUNLEVELS_DEFAULT
@@ -156,3 +157,4 @@ SUBSYSTEM_DEF(lua)
 			continue
 		state.log_result(json_data)
 	return
+#endif