feat!: emit warning that StrictUndefined will be enabled by default in the future

[mschoettle]

BREAKING CHANGES: Existing templates will see a FutureWarning emitted.

Follow-up to #2587 as a second phase towards enabling StrictUndefined by default.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.16%. Comparing base (e05ddfe) to head (2df3930).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2590   +/-   ##
=======================================
  Coverage   97.15%   97.16%           
=======================================
  Files          60       60           
  Lines        7101     7117   +16     
=======================================
+ Hits         6899     6915   +16     
  Misses        202      202           

Flag	Coverage Δ
unittests	97.16% <100.00%> (+<0.01%)	⬆️

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[mschoettle]

@sisp Before I put more effort into this. Is there any interest in having this change (even if it is not merged right away)?

[sisp]

@mschoettle I think eventually we'll want to use StrictUndefined by default because it's the safer behavior, but #2482 is holding me back from even announcing the future change at this point. I'd leave it up you whether you'd like to update this PR and keep it open until we've resolved #2482 or close it in favor of an issue that tracks it with a reference to resolving #2482 as a precondition.

[mschoettle]

@sisp I brought it up to date and will leave it open. Is there anything I can help to advance #2482?

[sisp]

Thanks for your efforts and offer to help advance #2482, @mschoettle! 🙏 The way I see it, #2482 is best addressed after a major refactoring that decouples prompting from generation better. I've been working towards it with as much time as I can dedicate, hopefully it will be ready soon-ish. 🤞 I anticipate that implementing this fix and several other enhancements will become much easier then. I realize this is a somewhat unsatisfying, somewhat vague reply, but I'm convinced the wait will be worth it. 💪 🙂