vim-patch:9.1.0995: filetype: shaderslang files are not detected

Problem:  filetype: shaderslang files are not detected
Solution: detect '*.slang' files as shaderslang filetype,
          include a filetype and syntax script (mtvare6)

Reference:
https://shader-slang.com/

closes: vim/vim#16387

616219f684

Co-authored-by: mtvare6 <mtvare6@proton.me>

runtime/lua/vim/filetype.lua

@@ -1097,6 +1097,7 @@ local extension = {
   la = 'sh',
   lai = 'sh',
   mdd = 'sh',
+  slang = 'shaderslang',
   sieve = 'sieve',
   siv = 'sieve',
   sig = detect.sig,