Add LSP support to upstream CMake

It would be useful for upstream CMake to provide an LSP server, so it can integrate with IDEs.

Regular features would be:

• Follow symbol / go to declaration (targets, variables, properties, commands, cmake module files, Find scripts, found find_package() Config files )
• Find usages (variables, commands, targets for known commands)
• Rename variable / command / property
• Code auto completion (variable, property, command names, options for known commands)
• Hover documentation (well known commands, variables, properties)

Nice to have features:

• Code formatting

Nice to have features that would perhaps use a custom extension to the LSP protocol:

• Source file addition to add_executable / target_sources calls (even custom commands, like qt_add_executable / qt_add_qml_module / add_jar)