Experiment with better snapping defaults while in `drawMode`

[jessicamcinchak]

Current implementation: https://github.com/theopensystemslab/map/blob/main/src/draw.ts#L46-L49

Can we additionally snap drawings to OS Vector Tiles basemap or Features layer if/when they're enabled?

Docs:

• https://openlayers.org/en/latest/apidoc/module-ol_interaction_Snap-Snap.html

Examples:

• https://openlayers.org/en/latest/examples/snap.html
• http://dev.openlayers.org/examples/snapping.html
• TBD examples from Bucks & Lambeth internal tools

[jessicamcinchak]

Snap to OS base map layer