Support of MapLibre style specification

[maxammann]

This discussion was initiated by @nyurik:

I understand the desire to innovate and revisit the boring old specs,and I even (in part) agree, but we should also consider existing users and their ecosystems, and huge investment many companies have already done into designing their own maps. If maplibre-rs is to become the awesome new tech we all want it to be, there should be an easy path to adaption - and that means minimal migration pain, instead of rewriting all styling from scratch. I would recommend (eventually) implementing full support of the current styling language, possibly obsoleting a few bad design decisions if we can prove they were bad. If needed, we could have a new styling language plus some optional compatibility layer - but again, must show we are breaking it for a good reason.