Fix selection in readOnly mode, Add magnifier via RawMagnifier widget

[mtallenca]

Description

1. Allow selection in read only mode
2. Displays magnifier when dragging on iOS/Android

Related Issues

• Fix Can't select text when readOnly to true #2518
• Fix Text magnification feature for the Quill Editor #1504

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.

[CatHood0]

From what I see, this restores the magnifier behavior, right?

We've had issues with this implementation before, and it led to some problems during rollbacks.

For the reasons mentioned above, it's best to test this PR until we know it won't cause us problems, like it did last time.

In any case, thank you for taking the time and effort to make these changes.

[mtallenca]

This does restore magnifier behavior. I did not create the previous magnifier implementation, but I did fix some of it's bugs that had been merged. However, this is a completely different implementation that is using Flutter's RawMagnifier widget in the Widget tree (see text_selection.dart) instead of changing events / config / etc throughout the editor. The other dart changes in this PR are just passing a ValueNotifier around, and that ValueNotifier is only used in text_selection.dart.

(This PR also has some selection fixes when in read only mode)

I agree needs to be tested. I have an internal team of 8, 1.2 million app users creating just over 10K notes a day. They have been running the 10.8.5 version that had my previous magnifier fixes. Prior to the fixes, we received several support tickets about the magnifier bugs, since the fixes no reports.

After we finish internal testing, we will do a rollout to our users and the feature will be used there. We have finished updating to flutter_quill 11.x and require the selection and magnification changes.

[EchoEllet]

As a minor note, we should update the v11 migration guide to inform clients that the magnifier has been restored in version X after it was removed. This is not part of this PR since it should be done after releasing the new version.