[WIP] Leaflet 2 compatibility

[evnchn]

Motivation

To pave the way to Leaflet's official 2.0.0 release, seeing https://github.com/Leaflet/Leaflet/releases/tag/v2.0.0-alpha

DO NOT MERGE INTO MAIN!!! Merge into another branch only, or just look at it here.

Implementation

• Use the global version and apply polyfill for maximum compatibility
• Update all the assets

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

Works (as in the NiceGUi documentation):

• Zoom and move
• Changing map style
• Add markers on click
• Move markers
• Image overlays, Video overlays
• Disable pan and zoom
• Run map methods
• Run layer methods
• Wait for initialization
• Leaflet plugins

Doesn't work (as in the NiceGUi documentation):

• Vector layers
• Draw on map
• Dynamically load in the Leaflet maps which are off-screen and became on-screen with scrolling (,_ this is a big one)

[evnchn]

We'll take another shot after NiceGUI 3.0