runtime(sml): add filetype plugin, move options from indent to ftplugin

closes: #18680

Signed-off-by: tocariimaa <tocariimaa@pissmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/sml.vim

@@ -0,0 +1,39 @@
+" Vim filetype plugin file
+" Language: 		SML
+" Filenames:		*.sml *.sig
+" Maintainer: 		tocariimaa <tocariimaa@firemail.cc>
+" Last Change:		2025 Nov 04
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+let s:cpo_save = &cpo
+set cpo&vim
+
+let b:undo_ftplugin = 'setl com< cms< fo<'
+
+setlocal formatoptions+=croql formatoptions-=t
+setlocal commentstring=(*\ %s\ *)
+setlocal comments=sr:(*,mb:*,ex:*)
+
+if exists('loaded_matchit')
+  let b:match_ignorecase = 0
+  let b:match_words = '\<\%(abstype\|let\|local\|sig\|struct\)\>:\<\%(in\|with\)\>:\<end\>'
+  let b:undo_ftplugin ..= ' | unlet! b:match_ignorecase b:match_words'
+endif
+
+if (has("gui_win32") || has("gui_gtk")) && !exists("b:browsefilter")
+  let b:browsefilter = "SML Source Files (*.sml)\t*.sml\n" ..
+                     \ "SML Signature Files (*.sig)\t*.sig\n"
+  if has("win32")
+    let b:browsefilter ..= "All Files (*.*)\t*\n"
+  else
+    let b:browsefilter ..= "All Files (*)\t*\n"
+  endif
+  let b:undo_ftplugin ..= " | unlet! b:browsefilter"
+endif
+
+let &cpo = s:cpo_save
+unlet s:cpo_save