feat(editorconfig): add editorconfig syntax file

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.

runtime/lua/vim/filetype.lua

@@ -1378,13 +1378,13 @@ local filename = {
   npmrc = 'dosini',
   ['/etc/yum.conf'] = 'dosini',
   ['.npmrc'] = 'dosini',
-  ['.editorconfig'] = 'dosini',
   ['/etc/pacman.conf'] = 'confini',
   ['mpv.conf'] = 'confini',
   dune = 'dune',
   jbuild = 'dune',
   ['dune-workspace'] = 'dune',
   ['dune-project'] = 'dune',
+  ['.editorconfig'] = 'editorconfig',
   ['elinks.conf'] = 'elinks',
   ['mix.lock'] = 'elixir',
   ['filter-rules'] = 'elmfilt',