Minimal pmtiles http support

[acalcutt]

This adds minimal pmtiles support to the web plugin as requested in #212 . It is a lite version of the changes I made in https://github.com/acalcutt/maplibre-contour-pmtiles/ without the local pmtiles support.

-Changes-
Adds support for pmtiles:// urls, so these are valid urls now

https://url/of/dem/source/{z}/{x}/{y}.png

pmtiles://https://url/of/dem/source.pmtiles

In #359 support was added to add a GetTile function, but this function only was passing url, which made it hard to use for things that needed the zxy values, since you had to split them out of the url. I have modified this a bit some it now passes

(
    z: number,
    x: number,
    y: number,
    demUrlPattern: string,
    abortController: AbortController,
    pmtiles?: PMTiles | null,
  )

Tests have keen updated to use the new "cacheKey" value instead of url. The GetTile mock function also had to be changed since different values are now passed.