runtime/elm: acc224064033e5cea21ef7f1eefb356ca06ff11d (#13718)

Port ftplugin, syntax and indent Elm files.

Ref: #13193

runtime/ftplugin/elm.vim

@@ -0,0 +1,18 @@
+" Elm filetype plugin file
+" Language: Elm
+" Maintainer: Andreas Scharf <as@99n.de>
+" Latest Revision: 2020-05-29
+
+if exists("b:did_ftplugin")
+  finish
+endif
+let b:did_ftplugin = 1
+
+let s:cpo_save = &cpo
+set cpo&vim
+
+setlocal comments=s1fl:{-,mb:\ ,ex:-},:--
+setlocal commentstring=--\ %s
+
+let &cpo = s:cpo_save
+unlet s:cpo_save