chore: internalize FormBuilder

[yohanboniface]

fix #2280

That's a first step, which:

• internalize Formbuilder as a bunch of modules
• use Javascript classes instead of Leaflet ones
• remove dependencies to Leaflet (L.DomUtil…)
• replaces L.FormBuilder by Form (in theory generic, but not quite) and U.FormBuilder by MutatingForm (knows about isDirty, inheritable and such)

There is much more room for refactor, but let's do it step by step!

[davidbgk]

This is huge! And it paves the way to improve forms' accessibility 👍

I wonder if we could replace the callback mechanism, let's discuss that in particular before merging.

[yohanboniface]

I wonder if we could replace the callback mechanism, let's discuss that in particular before merging.

For sure! What kind API do you have in mind ?

[davidbgk]

For sure! What kind API do you have in mind ?

Not sure how we can deal with a Promise or dedicated events here (and maybe that's a bad idea).

[davidbgk]

All 🟢