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: #16387
Signed-off-by: mtvare6 <mtvare6@proton.me>
Signed-off-by: Christian Brabandt <cb@256bit.org>
