refactor!: restructure project into modular architecture for flutter_quill_extensions

[EchoEllet]

Description

This PR is identical to #2032 for flutter_quill_extensions

This will be a breaking change for users who import files directly, import the extensions package using:

import 'package:flutter_quill_extensions/flutter_quill_extensions.dart';

Some files are not exported and are available as public API.

So to use them you have to import the file and ignore the warning manually:

// ignore: implementation_imports
import 'package:flutter_quill_extensions/src/editor/image/widgets/image.dart'
    show getImageProviderByImageSource, imageFileExtensions;

Keep in mind the APIs that are not exposed in public are not stable, and could be removed or changed at any time.

This PR has other minor breaking changes.

The following APIs have been removed:

• QuillEditorUnknownEmbedBuilder: this embed/widget shows a text indicating this embed is unknown instead of showing an error. You can create your own and use unknownEmbedBuilder.
• The deprecated parameter mediaButtonOptions in the static function FlutterQuillEmbeds.toolbarButtons: since the media button has been removed for a while.

Related Issues

• Related #2032

Type of Change

• [ ] ✨ New feature: Adds new functionality without breaking existing features.
• [ ] 🛠️ Bug fix: Resolves an issue without altering current behavior.
• [x] 🧹 Code refactor: Code restructuring that does not affect behavior.
• [x] ❌ Breaking change: Alters existing functionality and requires updates.
• [ ] 🧪 Tests: Adds new tests or modifies existing tests.
• [ ] 📝 Documentation: Updates or additions to documentation.
• [ ] 🗑️ Chore: Routine tasks, or maintenance.
• [ ] ✅ Build configuration change: Changes to build or deploy processes.