WebView plugin: Javascript channel names can't contain underscores, even though the documentation says so

Internal: b/135906327

They can start with an underscore but the rest of the name can't contain them.

Code: https://github.com/flutter/plugins/blob/410095a1bae31e888d0f9d668510190c9eda5102/packages/webview_flutter/lib/webview_flutter.dart#L75