Migrate libphonenumber_web to Next-generation JS interop

[Kal-Elx]

Closes #38.

I encountered this issue while trying to build a Flutter web app using WebAssembly, which includes intl_phone_number_input as a dependency—and thus, this plugin as a transitive dependency.

I ran a migration to use next-generation JS interop using gemini-2.5-pro, which resolved the issue.

I'm overriding the dependency as follows:

dependency_overrides:
  libphonenumber_web:
    git:
      url: https://github.com/Kal-Elx/plugin_libphonenumber.git
      ref: migrate_to_next_gen_js_interop
      path: libphonenumber_web

I've verified that the changes work by running the example project in intl_phone_number_input.

This is the extent of the testing I’ve done. I'm confident including it in my app, but for further quality assurance, I defer to the maintainers of this package.