Introduce a new configuration to select a kit automatically if it exists. This feature is generally useful for the devcontainer usage.

Use cases
1. The specified kit is tried to be selected internally by using the CMake extension commands.
2. If auto is used and there is only one Qt kit, `qt-cpp` selects that kit. Otherwise, a message is sent to the user that auto only works when only one kit exists.
3. If a kit is explicitly selected by the user, qt-cpp.selectKit is not used at all.