Currently the plugin template uses platform channels as the cross-platform API, including web. We should change it to use a platform interface internally, just as we would for a federated plugin; there's no reason that platform interfaces need federation, and adding it would:

• make it clearer that platform channels don't have to be the cross-platform interface
• demonstrate a better pattern for web
• make it clearer how to, e.g., use FFI for one implementation
• be more consistent with the eventual federated template, so the two templates are more similar.

/cc @ditman