Bracketed escaping

[jayvdb]

Adds {\}, {{} and {}} as an escaping mechanism in settings.

{{} and {}} are not implemented so they can be used safely together.
Additional coding is needed to handle values from the engine being pushed
back into the engine, so they are re-escaped.

Fixes #1708

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword
  in message body
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[jayvdb]

highlighting this as what fails with the incomplete implementation.

[gaborbernat]

Adds {\}, {{} and {}} as an escaping mechanism in settings.

{{} and {}} are not implemented so they can be used safely together.
Additional coding is needed to handle values from the engine being pushed
back into the engine, so they are re-escape

I would prefer not to introduce new ways of doing things. As such only \ character I'm interested in supporting, I don't agree with supporting {{ or }} 🤔

[jayvdb]

I don't agree with supporting {{ or }}

I am also not convinced about adding those; I've tried to lay out the problems at

#1708

That commit in this PR is mostly to show where the problem exists, so anyone can play with it if they think it is easy to solve.

[gaborbernat]

This now has merge conflicts.

[gaborbernat]

I'll close this for now until you can attend to the PR again. Once you've done we can reopen.