add syntax highlight for CMakeCache.txt

Fix pboettch#28

ftdetect/cmake.vim

@@ -0,0 +1,5 @@
+augroup cmake
+  autocmd!
+  autocmd BufNewFile,BufRead CMakeLists.txt,*.cmake setfiletype cmake
+  autocmd BufNewFile,BufRead CMakeCache.txt setfiletype cmakecache
+augroup END

syntax/cmakecache.vim

@@ -0,0 +1,30 @@
+" Vim syntax file
+" Program:      CMake - Cross-Platform Makefile Generator
+" Version:      cmake version 3.27.20230713-gdc88dd5
+" Language:     CMake
+" Author:       Andy Cedilnik <[email protected]>,
+"               Nicholas Hutchinson <[email protected]>,
+"               Patrick Boettcher <[email protected]>
+" Maintainer:   Dimitri Merejkowsky <[email protected]>
+" Former Maintainer: Karthik Krishnan <[email protected]>
+" Last Change:  2023 Jul 13
+"
+" Licence:      The CMake license applies to this file. See
+"               https://cmake.org/licensing
+"               This implies that distribution with Vim is allowed
+
+if exists('b:current_syntax')
+  finish
+endif
+let b:current_syntax = 'cmakecache'
+
+syntax region cmakecacheComment start="#" end="$"
+syntax region cmakecacheComment start="//" end="$"
+syntax match cmakecacheKeyword /^\S*\ze:/
+syntax match cmakecacheType /:\zs\S*\ze=/
+syntax match cmakecacheConstant /=\zs.*/
+
+highlight default link cmakecacheComment Comment
+highlight default link cmakecacheKeyword Keyword
+highlight default link cmakecacheType Type
+highlight default link cmakecacheConstant Constant