feat: provide snapping to line as well as feature coordinates for polygon mode

[JamesLMilner]

Description of Changes

This PR sets up some breaking API changes for snapping, which paves the way for allowing snapping to lines as well as coordinates for the existing polygon mode features. Line and coordinate snapping can be enabled/disabled individually.

• Change polygon mode snapping property from optional boolean to optional object of type { toLine?: boolean; toCoordinate?: boolean; }
• Provides toLine which allows snapping to the line segments of polygon mode features
• Change linestring mode snapping property from optional boolean to optional object of type { toCoordinate?: boolean; }
• Fixes bug with initial snapping of linestrings on the first coordinate click
• Add the CartesianPoint type to standardised x, y coordinates across the codebase

Link to Issue

#307

PR Checklist

• The PR title follows the conventional commit standard
• There is a associated GitHub issue
• If I have added significant code changes, there are relevant tests
• If there are behaviour changes these are documented