vim-patch:9.1.0108: filetype: no support for dtso files

Problem:  filetype: no support for dtso files
Solution: Add detection for *.dtso files as dts file type
          (Markus Schneider-Pargmann)

*.dtso files are devicetree overlay files which have the same syntax as dts or dtsi files.

closes: vim/vim#14026

b1700fb33f

Co-authored-by: Markus Schneider-Pargmann <msp@baylibre.com>

runtime/lua/vim/filetype.lua

@@ -364,6 +364,7 @@ local extension = {
   d = detect.dtrace,
   dts = 'dts',
   dtsi = 'dts',
+  dtso = 'dts',
   dylan = 'dylan',
   intr = 'dylanintr',
   lid = 'dylanlid',