patch 9.1.0778: filetype: lf config files are not recognized

Problem:  filetype: lf config files are not recognized
Solution: detect lfrc files as lf filetype, include a syntax
          script for lf files (Andis Spriņķis).

References:
- https://github.com/gokcehan/lf

closes: #15859

Signed-off-by: Andis Spriņķis <spr.andis@protonmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/filetype.vim

@@ -1315,6 +1315,9 @@ au BufNewFile,BufRead *.lean			setf lean
 " Ledger
 au BufRead,BufNewFile *.ldg,*.ledger,*.journal			setf ledger
 
+" lf configuration (lfrc)
+au BufNewFile,BufRead lfrc			setf lf
+
 " Less
 au BufNewFile,BufRead *.less			setf less