fix(web): ensure the usage of `maplibre-gl-js` version 4.x.x, remove `_addStylesheetToShadowRoot`

[josxha]

• At the moment the package uses the latest maplibre-gl-js version in the example app. This could introduce unwanted breaking changes to the example app. This pull request limits the version to the latest version 4 release (4.x.x).
• The web package is using still v1.13.0-rc.4 of mapbox-gl.css from cdn.maptiler.com internally. This code has been added in an early stage of flutter web (maybe as a workaround in the early stage of flutter web?). The example app works the same without it, so I guess it's no longer needed and therefore removed in this pull request.
  • Update: This code was added because of https://github.com/flutter-mapbox-gl/maps/issues/514
• The web project files have significantly changed a lot since they were generated. This pull request recreates the web files of the example project.

[josxha]

I reverted the recreation of the example web project.