Add geolocation adapter for customizing geolocation in GeolocateControl

[mourner]

This is a rebase of #10400 with Flow / unit tests fixed — that PR got stuck not reporting CI for some reason.

const myGeolocation = {
   clearWatch: ...,
   getCurrentPosition: ...,
   watchPosition: ...,
};

map.addControl(new GeolocateControl({
   geolocation: myGeolocation
}));

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Add an option for providing a geolocation adapter to GeolocateControl</changelog>