[Task] Add support for input validation within input textarea

[tjtanjin]

Description: Several users have brought up request for feature of allowing input validation to be done within the input textarea. Currently, the only way to do validation is after user sends the message. It would be great if validation can be done before a user even sends it. The tentative suggested approach is as follows:

• Add a new inputValidator attribute to block which accepts 2 properties validator and errorMessage
• The validator property will return true or false and if false, then the errorMessage is displayed

Position of the error message is tentatively above chat input container in the form of a small popup bar. The chat input textarea itself should also be outlined in red when error is shown. Upon displaying the error, it should only be dismissed when user make changes to the textarea.

[Vince-ALIEN]

It's done, I think you're going to scream...

https://github.com/Vince-ALIEN/wp-chatbotify