refactor: restructure project into modular architecture for flutter_quill

[EchoEllet]

Description

Use the modular structure for flutter_quill to improve maintainability.

Our current project structure almost has no architecture, which was suitable when the project first started out. This is a partial fix.

Note

Notice this is not final and we should improve it further, by moving and splitting functionalities instead of having one big file.
There might be inconsistency or some minor issues, misleading, or further improvements.
In case I have any examples I would already fix it, for example, there might be some files in a module that should be in another module for clarity. Or it might be in the wrong directory.
We will improve anything later. For now, this structure can be easier to maintain than the current one.

The new project structure (not final and might be changed):

$ tree -d ./lib/
./lib/
└── src
    ├── common
    │   ├── extensions
    │   ├── structs
    │   └── utils
    ├── controller
    ├── delta
    ├── document
    │   ├── nodes
    │   └── structs
    ├── editor
    │   ├── config
    │   │   └── elements
    │   │       └── list
    │   ├── embed
    │   ├── raw_editor
    │   │   └── config
    │   ├── style_widgets
    │   └── widgets
    │       └── text
    ├── editor_toolbar_controller_shared
    │   └── clipboard
    ├── editor_toolbar_shared
    │   └── config
    ├── l10n
    │   ├── extensions
    │   ├── generated
    │   └── widgets
    ├── packages
    │   └── quill_markdown
    ├── rules
    └── toolbar
        ├── base_button
        ├── buttons
        │   ├── alignment
        │   ├── color
        │   ├── hearder_style
        │   └── search
        │       └── legacy
        ├── config
        │   └── buttons
        ├── embed
        ├── structs
        └── theme

We're open to all feedback.

Related Issues

• Fix Refactor the project structure without breaking changes #2031

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the package version in pubspec.yaml files.
• All existing and new tests are passing.
• I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

This change is a breaking change if you import files directly from the src directory:

import 'package:flutter_quill/src/editor_toolbar_controller_shared/clipboard/clipboard_service_provider.dart';

By default, dart analysis will give you a warning and the IDE will not import or suggest from the src directory unless you explicitly import.

flutter_quill_extensions is no longer compatible with an older version of flutter_quill, we use the same version for all packages on this repo for automated publishing and to improve compatibility.