Provide layer key as snap option

[dbauszus-glx]

Instead of snap:true it should be possible to provide a layer key.

"polygon": {
  "snap": {
    "layer": "iuc"
  }
}

https://github.com/GEOLYTIX/xyz/wiki/Workspace-Configuration#snap

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

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[simon-leech]

@dbauszus-glx I noticed if you provide a bogus layer key you get no warning or error, and the layer itself is defined as the snap layer. I think this should be explicit with a warning, so I pushed a commit to do this - let me know what you think.

The functionality is all working for me though!