Test adding contour lines

[acalcutt]

This is a test I did of adding contour lines on top of your recent PR. is is nowhere near complete, but I wanted to see if you had any opinions on it.

This uses you elevation endpoint as an example and makes one for contour lines
'^/:id/contour/:z([0-9]+)/:x([-.0-9]+)/:y([-.0-9]+)',

The contour endpoint generates vector tiles using a new feature of maplibre contour which allows you to overwrite the fetch tiles and decode image functions onthegomap/maplibre-contour#359 . I have made functions to work with that feature