Add point and polyline support for mapbox vector tile items using a library

[tephenavies]

Currently Terria only supports Polygons through MapboxVectorTileImageryProvider.js but now that we allow adding a catalog item with type "mvt" we should expand support.

[AnaBelgun]

to discuss complexity in sprint

[tephenavies]

I've prototyped this for points:

Unfortunately my approach has drawbacks for points. E.g. zooming in leads to artifacts (see to the left and below the point):

Although I think this is also a problem with the current polygon rendering, it just doesn't show up as much.

It'd be trivial to do render polylines too (and shouldn't lead to other problems)

[wibily]

Nick: this will be replaced by a library at some point.

[nf-s]

This is the lib I have my eye on https://github.com/protomaps/protomaps.js

[nf-s]

Closed by #5736