Fix asset loading for Flutter web

[tigloo]

This pull request makes two changes:

1. It updates the platform code so that included assets can be found. I'm not sure why the paths are hardcoded in the platform code - since we pass in the rootBundle during initialization, we should deduce asset location from the bundle. I've kept the code as is to not make any breaking changes.
2. Include the tzdb and cultures data directly in the package's pubspec.yaml file because the only way to include them in the app's pubspec file is by either adding a manual copy step in the CI/CD pipeline or manually listing every file. This fixes Flutter web release builds missing data #25 and tzdb.json is loaded incorrectly in flutter web release #64.

[followthemoney1]

ins case someone have the same issue after flutter upgrade 3.16.+
FormatException: FormatException: SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
this should resolve the issue itself