migrate away from deprecated bower.json

[patrickp-rthinfo]

Bower is deprecated. Popular alternatives are yarn or npm "plus X" where X stands for additional solutions like browserify.
If we want to go with npm, we must create a package.json and publish a package on npmjs.com.

[Vectorial1024]

This package was recently modernized (ref Vectorial1024#4).

The modernized package uses NPM + Rollup, and is available at https://www.npmjs.com/package/@vectorial1024/leaflet-color-markers . Most notably, the package works with the include syntax (more info in the README).

Users of both pointhi/leaflet-color-markers (not published on NPM) and patrickp-rthinfo/leaflet-color-markers (published as leaflet-color-markers) should migrate to use the modernized package @vectorial1024/leaflet-color-markers.