Add output parser for include-what-you-use

[malsyned]

This changes visible behavior & documentation

The following changes are proposed:

• Parse include-what-you-use output to generate warning and note diagnostics about missing or superfluous #includes and forward declarations.
• Add iwyu item to the cmake.enabledOutputParsers option.

The purpose of this change

To add diagnostic messages from include-what-you-use to the Problems pane.

Other Notes/Information

include-what-you-use is a C/C++ code linting tool built with LLVM. CMake has built-in integration via the CMAKE_<LANG>_INCLUDE_WHAT_YOU_USE variable and <LANG>_INCLUDE_WHAT_YOU_USE property.

I didn't add iwyu to the default value of cmake.enabledOutputParsers but I don't think there is any harm in doing so. Its diagnostic messages are quite distinctive and unlike those of the already-supported compilers and linkers. I'd be very open to modifying this PR to make it a default.