Add support for alternative tiles with different resolutions

[zdila]

Currently it is possible to tell Leaflet to serve tiles of specific resolution by computing it from window.devicePixelRatio and inserting it to TileLayer URL (see #6449 (comment)).

It would be nice to be able to provide list of additional supported scales (eg. [2, 3] for 2x and 3x ) and tiles represented by imgs would then use srcset attribute to tell browser which image to fetch depending on device resolution.

For an example you can see the implementation using react-leaflet in ScaledTileLayer.tsx that extends TileLayer and trivially adds srcset to img returned by createTile. The result can be seen at https://www.freemap.sk/?map=15/48.855595/20.941937&layers=X - view it on HDPI device (eg. mobile phone) or zoom the page in Firefox (Ctrl + +).