Reference leaflet as peer dependency instead of hard dependency

Currently Leaflet is referenced as `dependency`.
https://github.com/makinacorpus/Leaflet.GeometryUtil/blob/f2c09bcaf7db146a4c860b66af9724248fb0ed96/package.json#L23-L25

To allow more flexible dependency resolution, it should be declared as peer dependency:
```json
    "peerDependencies": {
        "leaflet": ">=1.6.0 <2.0.0"
    }
```