Add new handler: `src/handlers/runner.js`

[Raruto]

Extract some code from: examples/leaflet-elevation_dynamic-runner.html and move into src/handlers/runner.js

Sample usage:

L.control.Elevation({
  handlers: [ 'Runner' ], // same as: import('https://unpkg.com/@raruto/leaflet-elevation/src/handlers/runner.js')
  runnerOptions: {
    polyline: { .. }, // L.Motion.Polyline control options
    motion: { .. },   // L.Motion.Polyline motion options
    marker: { .. }    // L.Motion.Polyline marker options
  },
});

controlElevation.on('eledata_loaded', ({ layer }) => {
  controlElevation.animate(layer, 1500);
});

Refer to Igor-Vladyka/leaflet.motion#options see for a more comprehensive list of available runnerOptions.

Related to: https://github.com/Raruto/leaflet-elevation/issues/215

[Raruto]

@bard0x here it is a sample integration, let me know if there's something wrong.