Feat: include spell checker for example app

Description

Added the Spell-checker to the sample app which can be turned on or off at the user's discretion.

However, not only this was done but some code parts were added and fixed:

Added information about the Feature in the README
Fixed a bug where the editor placeholder is detected as text instead of virtual text which should be ignored
Updated simple_spell_checker to the latest version to fix issues with state and characters being lost when typing in the editor

[x] ✨ 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.
[x] 📝 Documentation: Updates or additions to documentation.
[x] 🗑️ Chore: Routine tasks, or maintenance.
[ ] ✅ Build configuration change: Changes to build or deploy processes.