feat: export and render building:part

[c84c]

First of all, thanks for this great project!

With this PR, I add the ability to export and render building:part relations (thanks to Multipolygon relations are supported natively by tilemaker).

For the 3D building render style, credits to @tgdbepe4 and his great style!

Resolve #458.

The result is shown below for 2 monuments in Rome (Italy).

Building	Before PR	After PR
"name:it": "Castel Sant'Angelo"
"name:it": "Piazza San Pietro"

[c84c]

I change formatting minimizing code changes

[pnorman]

OpenMapTiles says it just uses building=*, but later on goes to talk about the height of the building or building:part. I assume it's contradicting itself and in practice and the most common implementation includes building:part?

[c84c]

Exploring implementation they use also building:part to insert buildings in mbtiles.