Add version selection in Contao Theme distribution via type contao-theme

[MDevster]

Description

It would be nice if you could choose a theme version when you select a Contao theme. Currently, for example, a Contao 4.13 is installed by the Contao demo, my expectation would have been that it is at least a 5.3. It would be better if you could choose the version directly. Alternatively, as a theme provider you have to provide a repository for each available version. Or have I overlooked something?