Add intermediate hl-Group target for "cmakeKW…" hl-Groups

[adrian5]

Currently, all cmakeKW… highlight groups link to ModeMsg.

It would be good if these instead targeted a common (virtual) hl-Group, e.g. cmakeKW, which can then be targeted by colorschemes independently of the unrelated ModeMsg highlight.

This approach isn't uncommon among other syntax files, e.g. in css.vim:

hi def link cssAnimationProp cssProp
hi def link cssBackgroundProp cssProp
hi def link cssBorderProp cssProp
hi def link cssBoxProp cssProp
" etc...

hi def link cssAnimationAttr cssAttr
hi def link cssBackgroundAttr cssAttr
hi def link cssBorderAttr cssAttr
hi def link cssBoxAttr cssAttr
" etc...

For cmake.vim that would mean:

hi def link cmakeKWExternalProject cmakeKW
hi def link cmakeKWadd_compile_definitions cmakeKW
hi def link cmakeKWadd_compile_options cmakeKW
hi def link cmakeKWadd_custom_command cmakeKW
" etc...

" Same default style as before, but colorschemes can now override cmakeKW independently of ModeMsg
hi def link cmakeKW ModeMsg 

[pboettch]

Feel free to make a PR for this request.