patch 8.2.4747: no filetype override for .sys files

Problem:    No filetype override for .sys files.
Solution:   Add g:filetype_sys. (Patrick Meiser-Knosowski, closes #10181)

runtime/autoload/dist/ft.vim

@@ -819,7 +819,9 @@ export def FTperl(): number
 enddef
 
 export def FTsys()
-  if IsRapid()
+  if exists("g:filetype_sys")
+    exe "setf " .. g:filetype_sys
+  elseif IsRapid()
     setf rapid
   else
     setf bat

runtime/doc/filetype.txt

@@ -155,6 +155,7 @@ variables can be used to overrule the filetype used for certain extensions:
 	*.pp		g:filetype_pp	|ft-pascal-syntax|
 	*.prg		g:filetype_prg
 	*.src		g:filetype_src
+	*.sys		g:filetype_sys
 	*.sh		g:bash_is_sh	|ft-sh-syntax|
 	*.tex		g:tex_flavor	|ft-tex-plugin|
 	*.w		g:filetype_w	|ft-cweb-syntax|