Support readOnly property from latest validation draft

[pahen]

Reasons for making this change

Since the readOnly property now is available in the latest validation draft (previously it was in hyper schema only), it makes sense to use this property to signal read only for fields.

Fixes #976
Replaces PR #888

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[epicfaace]

Thanks!

[pahen]

Would it be possible to have a new release with this included soon?

[pahen]

Is there any plan for a new release soon @epicfaace? I really need this feature.