`markAvailable` - marks all the available magnets or elements when a link is dragged (being reconnected). Default is `false`. This gives a hint to the user to what other elements/ports this link can be connected. What magnets/cells are available is determined by the `validateConnection` function. Internally, available magnets (SVG elements) are given the `'available-magnet'` class name and all the available cells the `'available-cell'` class name. This allows you to change the styling of the highlight effect.