manually restored leaflet-providers-1.9.0.js

For some reason when I deleted `docs/assets/htmlwidgets` and did a build, it restored all the javascript files except `leaflet-providers-1.9.0.js` in the top-level directory. I'm not sure if this was needed but I manually restored that file using `git restore` before rebuilding, just in case it's needed. Leaving this issue as a comment in case we need to look into it after CSI