cmake.xml: Improvements to highlighting

• Fix variable names recognition. CMake syntax allows variable names to contains ., +, -, and even /.

• Fix target names recognition. Target names may contain +, -, ..

• Introduce Targets attribute to highlight targets where expected. Many CMake built-ins can have a target name as

  • the first parameter
  • leading parameters before first named argument
  • the next argument of a named argument (e.g., TARGET)
  • multiple arguments after a named argument (e.g., TARGETS)