LibCMake: Add a lexer for CMakeCache.txt

This is a totally different syntax than for regular CMake files, and
also is undocumented and subject to change, but it's also nice and
simple. :^)

Userland/Libraries/LibCMake/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(SOURCES
+    CMakeCache/Lexer.cpp
     Lexer.cpp
     SyntaxHighlighter.cpp
     Token.cpp