Use geometric lengths for LineSliceHighlight plugin

[geographika]

Fixes an issue with geometries in EPSG:2157 highlights are several metres off when measured against reference data.

The plugin originally assumed geometry in Web Mercator EPSG:3857 and used ol.sphere.getLength(segment); to calculate lengths. This updates the plugin to use segment.getLength(); for projected geometries.

if (projection && projection.getUnits() === 'degrees') {
    return ol.sphere.getLength(segment, {
        projection: projection.getCode()
    });
}
if (projection && projection.getCode() === 'EPSG:3857') {
    return ol.sphere.getLength(segment, { projection: 'EPSG:3857' });
}
return segment.getLength();

Also adds a test suite for the plugin.