Print warning if not using either preview or revert DIP1000 switches

[atilaneves]

If we transition to enabling DIP1000 by default (which we want to do), users will have to add -preview=dip1000 to maintain the current behaviour, with possible code breakage for those who don't. This might be a problem in dependencies.

To gently nudge the transition, this PR prints out a deprecation warning so that users are encouraged to user either preview or revert switches explicitly. After a grace period we can deprecate -preview=dip1000.

Nothing changes for now except for the annoying message.

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Testing this PR locally

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dub run digger -- build "master + dmd#12159"

[Geod24]

I'd be much more comfortable with pushing for this switch if we made sure most libraries in Buildkite compiled with it.

[Geod24]

Ahem, this DIP is superseeded

[atilaneves]

Oops. I guess the link should be this then?

[MoonlightSentinel]

Sorry to hijack this PR but this will create yet another flag that requires two global members instead of one => #11705

[wilzbach]

Wouldn't a much better solution to only show deprecations if there's code that would be broken with the new default?
Otherwise, everyone will just ignore the warning and complain about breakage once it's the default.

[atilaneves]

Wouldn't a much better solution to only show deprecations if there's code that would be broken with the new default?

I hadn't considered that. Let me take a look at the code and see how to get it done. Thanks for the suggestion!

[dkorpel]

Closing this in favor of #12578