vim-patch:9.1.0109: filetype: no support for its files

Problem:  filetype: no support for its files
Solution: Add detection for *.its files as dts file type
          (Brandon Maier)

The '*.its' file type is for U-Boot Flattened Image Trees (FIT) which
use the flattened devicetree format.

See https://github.com/u-boot/u-boot/blob/master/doc/usage/fit/source_file_format.rst#terminology

closes: vim/vim#14037

cf1d65e060

Co-authored-by: Brandon Maier <brandon.maier@collins.com>

runtime/lua/vim/filetype.lua

@@ -365,6 +365,7 @@ local extension = {
   dts = 'dts',
   dtsi = 'dts',
   dtso = 'dts',
+  its = 'dts',
   dylan = 'dylan',
   intr = 'dylanintr',
   lid = 'dylanlid',