vim-patch:9.2.0860: filetype: xilinx design constraint files are not recognized (#41024)

Problem:  filetype: xilinx design constraint files are not recognized
Solution: Detect *.xdc files as tcl filetype (Wu, Zhenyu)

Reference:
https://docs.amd.com/r/en-US/ug903-vivado-using-constraints/About-XDC-Constraints

closes: vim/vim#20853

247a4cb45e

Co-authored-by: Wu, Zhenyu <wuzhenyu@ustc.edu>

runtime/lua/vim/filetype.lua

@@ -1323,6 +1323,7 @@ local extension = {
   itcl = 'tcl',
   tk = 'tcl',
   jacl = 'tcl',
+  xdc = 'tcl',
   tl = 'teal',
   templ = 'templ',
   tmpl = 'template',