Issue #2942651 by jwilson3, itamair, hswong3i: Manage Leaflet library...

Issue #2942651 by jwilson3, itamair, hswong3i: Manage Leaflet library dependency with Composer & Asset Packagist

This PR simplify Leaflet module library management with:

  - Remove `leaflet_markercluster` submodule, simply merge its JS
    library initialization logic to parent `leaflet.libraries.yml`
  - Remove locally vendored `js/leaflet` and
    `modules/leaflet_markercluster/js/leaflet_markercluster`, in order
    to avoid any Drupal.org related licensing issue
  - Update `leaflet.libraries.yml` to include libraries from
    `/libraries`, with install path generated by using
    https://asset-packagist.org


  - Simplify and debug Marker Cluster related
  - Update `README.md` for how to install corresponding JS libraries

Signed-off-by: Wong Hoi Sing Edison <hswong3i@pantarei-design.com>