Add %{Project:SourceDirName} or similar persistent, documented variable

While %{Project:Path} currently works as expected, relying on JavaScript-based manipulation of this internal and undocumented variable to extract a file system path component introduces unnecessary complexity and fragility.

A simple, persistent, and officially documented variable such as %{Project:SourceDirName} would improve clarity and maintainability in Qt Creator project configurations. This variable would ideally resolve to the basename of the project's path that is, the name of the directory containing the main project file (.pro, .qbs, etc.).