Add support for read only codebox.

[HarveyRDonnelly]

🤔 This is a New Feature.

🔗 Related issue link

Most editor-js tools support a read-only mode and so there is a desire for this tool to offer the same functionality.

💡 Background and solution

Added readOnly attribute to the constructor. Then added relevant logic to the render function to disable editability of code area when CodeBox.readOnly is true. Followed convention for other editor-js tools.

📝 Changelog

Language	Changelog
🇺🇸 English

☑️ Self Check before Merge

• ☑️ Doc is updated/provided or not needed
• ☑️ Demo/Example is updated/provided or not needed
• ☑️ Changelog is provided or not needed

[Xoshbin]

Thank you @HarveyRDonnelly @dev-juju When would this be merged please?