* Make `CharacterReader` internal.
And by extension, make the static classes which define extension methods on it internal.
To do this, I had to make Avalonia.Base's internals visible to Avalonia.Designer.HostApp, as that has a dependency on it, and exclude `StringCompatibilityExtensions` from there as the one from Avalonia.Base is now visible.
* Don't expose internals to HostApp.
Exposing Avalonia.Base's internals to Avalonia.Designer.HostApp caused an issue with the dependency on `StringCompatibilityExtensions` - because the hostapp is loaded into the target app and `StringCompatibilityExtensions` is conditionally compiled, you ended up with it being loaded into an app which does not have `StringCompatibilityExtensions`.
Instead, add `string` overloads of the internal APIs which take a `CharacterReader` so that they can be called even when `CharacterReader` isn't available.
* Update API diff.
Steven Kirk
3cc0275506