Clean up and modularize JavaScript (particularly MapLibre)

[walkerke]

I have had so much fun writing this package. It's been amazing to see my dream R package become a reality.

However, as I have added feature upon feature, the JavaScript files have become bloated, with several thousands of lines of (sometimes duplicated) code among them.

The MapLibre code in particular has some vestigial hacks from me trying to debug the perplexing basemap switcher issue.

I want to leave this here (as everything "works") but I think it would be helpful for longer-term maintenance of the package to modularize and clean up the JavaScript.

I'm open to suggestions for users who are experienced maintaining JS projects.