add -preview=dip1021 to Phobos makefile

[WalterBright]

time to integrate dip1021

https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1021.md

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8980"

[pbackus]

Since -preview=dip1021 is a breaking change, shouldn't it be ineligible for the base language edition?

[dkorpel]

I guess the goal is to test it more in the wild, and adding the flag here just adds restrictions to Phobos' code without affecting any user code.