fixed #14464

What it does

Allows to use three brackets for variables in prompts. No mixed formats are allows for consistency

How to test

There are automated test.
To manually test,

• open the code completion prompt and a code file.
• Add one bracket to {{textUntilCurrentPosition}}, see how it fails.
• Add the second bracket to complete {{{textUntilCurrentPosition}}} and see how it works again.
• Check that the variable is still listed in the AI configuration view then.
• Check that in the prompt editor, three brackets are valid, but mixtures are an error

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines