BREAKING CHANGE: This deprecates the IS-(MAC|WINDOWS|LINUX|BSD) family
of global constants in favor of a native `featurep` check:
IS-MAC -> (featurep :system 'macos)
IS-WINDOWS -> (featurep :system 'windows)
IS-LINUX -> (featurep :system 'linux)
IS-BSD -> (featurep :system 'bsd)
The constants will stick around until the v3 release so folks can still
use it -- and there are still some modules that use it, but I'll phase
those uses out gradually.
Fix: #7479
Moves this from doom-ui to doom-start, since there is more savings to be
had if this is done early.
Also moves the menu-bar fix for macos out of the :os macos module into
doom-start, because it is a fix (and for a Doom optimization) and not a
feature, so it shouldn't be behind a module.
Some users may not be using a theme (*shudder*), in which case
ns-auto-titlebar-mode never kicks in.
That said, this "fix" didn't fix the targeted issue anyhow.
If triggered too early, it may read the wrong frame background mode and
you'll get dark text on a dark titlebar in MacOS dark mode (or light
text on a light titlebar in light mode).
