Other plugins may define their own custom properties outside of Neovim's
builtin EditorConfig support. Instead of highlighting these unknown
properties as errors, do not highlight them at all.
This still differentiates between known and unknown properties, which
helps to catch typos or mistakes, but does not use the garish "error"
highlight that signals something is wrong.
This is intentionally _not_ copied from Vim because our syntax file
makes use of Lua to dynamically generate a list of valid EditorConfig
properties. This requires the builtin editorconfig module, which Vim
does not have.
