Add variable support with #{} syntax in prompttemplate

[eneufeld]

What it does

Add variable support with #{} syntax in prompttemplate

How to test

Open the prompt template editor, eg for custom editors and try to use a variable. Using #{ you get an autocompletion.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

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

[eneufeld]

I retested and the auto-completion works but the variables are not resolved. Furthermore this does not work in the default prompt for custom agents in the yml.
We need to align this with #14899 and finally also decide whether we will show ids or names for functions, variables and agents