patch 9.1.0823: filetype: Zephyr overlay files not recognized

Problem:  filetype: Zephyr overlay files not recognized
Solution: detect '*.overlay' files as dts filetype,
          include syntax tests for DTS files
          (Xudong Zheng)

Reference:
https://docs.zephyrproject.org/latest/build/dts/howtos.html

closes: #15963

Signed-off-by: Xudong Zheng <7pkvm5aw@slicealias.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_filetype.vim

@@ -239,7 +239,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     dracula: ['file.drac', 'file.drc', 'file.lvs', 'file.lpe', 'drac.file'],
     dtd: ['file.dtd'],
     dtrace: ['/usr/lib/dtrace/io.d'],
-    dts: ['file.dts', 'file.dtsi', 'file.dtso', 'file.its', 'file.keymap'],
+    dts: ['file.dts', 'file.dtsi', 'file.dtso', 'file.its', 'file.keymap', 'file.overlay'],
     dune: ['jbuild', 'dune', 'dune-project', 'dune-workspace', 'dune-file'],
     dylan: ['file.dylan'],
     dylanintr: ['file.intr'],

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    823,
 /**/
     822,
 /**/