Problem: TI linker files are not recognized
Solution: inspect '*.cmd' files and detect TI linker files
as 'lnk' filetype, include a lnk ftplugin and syntax
script (Wu, Zhenyu)
closes: vim/vim#1632039a4eb0b2c
Co-authored-by: Wu, Zhenyu <wuzhenyu@ustc.edu>