Doesn't support Composer 2

Mercator doesn't support Composer 2:

[ "composer/installers": "~1.0" ](https://github.com/humanmade/Mercator/blob/55e3f011759dae8de0045f5d8ef136949970e072/composer.json#L18)

We'd like to update our bedrocks to using Composer 2, but can't when they include Mercator as a dependency. Would you consider updating dependency to something like: "composer/installers": 1 - 2 or "composer/installers": "~1.0 || ~2.0"?