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/modules/admin/verbs/lua/helpers.dm

@@ -1,3 +1,4 @@
+#ifndef DISABLE_DREAMLUAU
 #define PROMISE_PENDING 0
 #define PROMISE_RESOLVED 1
 #define PROMISE_REJECTED 2
@@ -34,3 +35,4 @@
 #undef PROMISE_PENDING
 #undef PROMISE_RESOLVED
 #undef PROMISE_REJECTED
+#endif