feat(web): add support for OS-provided user dictionaries 📖

As originally listed on https://github.com/keymanapp/keyman/issues/5673:

OS integration:
- [ ] #3987
- [ ] #344

While we aren't doing predictive-text for those platforms quite yet, the sentiment is real and _is_ something we can address for Android and iOS:

- https://developer.android.com/guide/topics/providers/content-provider-basics#Basics
- For iOS: UILexicon apparently allows us to offer suggestions from user's local dictionary (contacts, etc). May be helpful. But yes, iOS so much more limited than Android.

This would allow us to include a user's contacts (names) and such as predictive-text suggestions. Names generally aren't in lexical models, yet can often be frequently used - as a result, this would be quite helpful to users.