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
Did I already mention that this `latex-mode` slash `LaTeX-mode`
ambiguity was a reeeeeeally bad decision upstream? Anyways, this very
small PR also loads `writegood-mode` with AucTeX, which uses
`LaTeX-mode`, and not `latex-mode`.
