vim-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: vim/vim#15963

a68bd6f089

Co-authored-by: Xudong Zheng <7pkvm5aw@slicealias.com>

runtime/lua/vim/filetype.lua

@@ -402,6 +402,7 @@ local extension = {
   dtso = 'dts',
   its = 'dts',
   keymap = 'dts',
+  overlay = 'dts',
   dylan = 'dylan',
   intr = 'dylanintr',
   lid = 'dylanlid',