Chore: Move spellchecker service to extensions

[CatHood0]

Description

Since SimpleSpellCheckerService is a custom implementation that is not needed by default, was moved to Flutter Quill Extensions and can be used such as SuperClipboard implementation:

FlutterQuillExtensions.useSpellCheckerService('<your_language>');

And was fixed some issue with some missing characters while typing (this is for languages such as portuguese, deutsch, italian, etc)

• ✨ New feature: Adds new functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Code refactor: Code restructuring that does not affect behavior.
• ❌ 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.

Suggestions

Please, after update the version of the main repo, also update it into flutter_quill_extensions to avoid conflicts since the new version change the names of the classes and methods.

[EchoEllet]

We might want to enable this service in the example as well.

[CatHood0]

I'll add it later. I'm still doing the documentation about the Feature to the README

[EchoEllet]

This can be a breaking change, though it hasn't been used yet.

I'm interested in knowing the reason for the naming changes if available.

Same for the static method setInstance

[CatHood0]

I changed the names so that they made more sense in the context of what the service is in charge of doing. However, as it is still an experimental feature for now, it is quite subject to change.

For example, with documents that have many spelling errors (if the Spell checker detects them) then the load could be greater than expected.

For this I am still looking for a solution to how Word or LibreOffice do it, which first show all the text, and then little by little let the UI update the specific parts that are wrong, which avoids that drop in performance of the app

Or that, for example, suggestions for words that are misspelled have not yet been implemented.

It is likely that I will soon change the names of the instances to be less confusing, but it would be in the update for Flutter Quill 11