Add option to highlight un-assigned areas

[nofurtherinformation]

Highlight areas that have not yet been assigned to a zone. Style should be something like:

...
'line-color': [
        'case',
        ['boolean', ['feature-state', 'focused'], false],
        '#000000', // Black color when focused
        ['boolean', ['feature-state', 'highlighted'], false],
        '#e5ff00', // yellow color when highlighted
        ['==', ['feature-state', 'zone'], null],
        '#e5ff00', // yellow color when zone is not assigned
        '#000000', // Default color
      ],
      'line-width': [
        'case',
        ['boolean', ['feature-state', 'focused'], false],
        5, // Width of 5 when focused
        ['boolean', ['feature-state', 'highlighted'], false],
        5, // Width of 5 when highlighted
        ['==', ['feature-state', 'zone'], null],
        5,
        0, // Default width
      ],
...

[nofurtherinformation]

Resolved in #173