[WIP] Deprecate typesafe variadic class arguments

[dkorpel]

Testing it as an error first to see if anything on buildkite relies on this feature.

[dlang-bot]

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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 + dmd#20673"

[thewilsonator]

backticks around class

[thewilsonator]

also, PR title says deprecation, this is an error

[dkorpel]

See PR description