feat(runtime): import cleanadd.vim from Vim (#17699)

Latest version from Vim v7.2a.
Closes https://github.com/neovim/neovim/issues/12875

runtime/spell/cleanadd.vim

@@ -0,0 +1,32 @@
+" Vim script to clean the ll.xxxxx.add files of commented out entries
+" Author:	Antonio Colombo, Bram Moolenaar
+" Last Update:	2008 Jun 3
+
+" Time in seconds after last time an ll.xxxxx.add file was updated
+" Default is one second.
+" If you invoke this script often set it to something bigger, e.g. 60 * 60
+" (one hour)
+if !exists("g:spell_clean_limit")
+  let g:spell_clean_limit = 1
+endif
+
+" Loop over all the runtime/spell/*.add files.
+" Delete all comment lines, except the ones starting with ##.
+for s:fname in split(globpath(&rtp, "spell/*.add"), "\n")
+  if filewritable(s:fname) && localtime() - getftime(s:fname) > g:spell_clean_limit
+    if exists('*fnameescape')
+      let s:f = fnameescape(s:fname)
+    else
+      let s:f = escape(s:fname, ' \|<')
+    endif
+    silent exe "tab split " . s:f
+    echo "Processing" s:f
+    silent! g/^#[^#]/d
+    silent update
+    close
+    unlet s:f
+  endif
+endfor
+unlet s:fname
+
+echo "Done"