Update MaplibreGL to the latest version

[dbrnz]

We are currently using 5.5.0 yet the latest version is 6.6.0.

As well as fixes and enhancements, the new version has stricter type checking so our code needs some tidying. See also issue #87.

[dbrnz]

Note: we need at least 5.12.0 to use Maplibre Tiles -- see https://maplibre.org/maplibre-tile-spec/implementation-status/.