Fix Bugzilla 21564 - assignment syntax for instantiating mixin templates

[ntrel]

Following the alias this change, I think mixin instantiation was the last place not using assignment syntax.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21564	enhancement	Allow assignment syntax for instantiating mixin templates

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 + dmd#16387"

[RazvanN7]

ping @ntrel

[thewilsonator]

https://github.com/dlang/dlang.org/pull/3808 somehow I missed this

[Bolpat]

@thewilsonator,

Looks good to me. Need […] a possibly a changelog entry

I think it does, so I added one: #16884