[FEATURE] Add support for public in-element

[simonihmig]

As per RFC287.

• This does not yet add API docs, will follow in a separate PR
• The removal of the hereby deprecated {{-in-element}} is slated for 4.0. The RFC suggested the next version after the next LTS, but given that the deprecation will be added at 3.19 at the earliest, I would think 4.0 seems appropriate (depending on how many more 3.x releases are expected)

[locks]

Thank you for the help! I don't think this is wrapped in a feature flag, so I would like to see API documentation be merged with the implementation so we make sure any release with the feature also contains the documentation. I can help with it!

[simonihmig]

@locks added a commit with the API docs!

[simonihmig]

Travis build itself is green, but still shown here as pending. 🤔

[pzuraq]

This looks good to me! Thanks for getting this done 😄

[pzuraq]

Ah, one thing actually, could you add a canary feature flag that guards this? Shouldn't be too hard, it should probably just re-enable the transform that errors for {{in-element}}

[simonihmig]

Ah, one thing actually, could you add a canary feature flag that guards this?

@pzuraq done. Hope I got the usage of feature flags right, please re-review!

[pzuraq]

I think it's probably fine to enable by default in this PR too, since it's not a major refactor. The feature flag is mostly to give us a way to turn it off in case we do run into major issues during beta. Will confirm though

Scratch that, we'll bring it up in the next core team meeting to enable, going through the standard process (checked with @rwjblue)

[simonihmig]

Ah right. Now that you are mentioning this, I remember that I wanted to ask about this 😝

[pzuraq]

Sorry, seems I was mistaken. Assuming travis passes I think this is good to go!

[simonihmig]

Travis seems to undergo maintenance, the build is not showing up here so far, but it is running here

[pzuraq]

Thanks again @simonihmig!