feat(linkTools.Button): allow distance to be defined via callback

[kumilingus]

Description

Allow the position of the link tool buttons to be dynamic.

For instance, based on the connection length:

new linkTools.Button({
    // show button 20px from the target for short links
    // and in the middle of the link for longer links
    distance: (view) => view.getConnectionLength() < 100 ? -20 : '50%'
});