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

vim/vim@cf1d65e

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',

test/old/testdir/test_filetype.vim

@@ -204,7 +204,7 @@ func s:GetFilenameChecks() abort
     \ 'dracula': ['file.drac', 'file.drc', 'filelvs', 'filelpe', 'drac.file', 'lpe', 'lvs', 'some-lpe', 'some-lvs'],
     \ 'dtd': ['file.dtd'],
     \ 'dtrace': ['/usr/lib/dtrace/io.d'],
-    \ 'dts': ['file.dts', 'file.dtsi', 'file.dtso'],
+    \ 'dts': ['file.dts', 'file.dtsi', 'file.dtso', 'file.its'],
     \ 'dune': ['jbuild', 'dune', 'dune-project', 'dune-workspace'],
     \ 'dylan': ['file.dylan'],
     \ 'dylanintr': ['file.intr'],