qt: add Vulkan/molten-vk support

[EndangeredNayla]

• Have you followed the guidelines for contributing?

• Have you ensured that your commits follow the commit style guide?

• Have you checked that there aren't other open pull requests for the same formula update/change?
  qt 6.4.1 #115926 exists but is also for something else, this PR just deals with Vulkan

• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?

• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?

• Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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[EndangeredNayla]

Im willing to disuss this further over IRC or whatever the homebrew community uses

[cho-m]

Previous attempts:

• qt: add Vulkan/molten-vk support #111474
• qt: add vulkan support #50038

Comments from last attempt still hold: #111474 (comment)

please test that your changes produce the intended result, and update the formula's test to include a minimal check for the usability of the new features.

[fxcoudert]

These have been incorporated, as well as a test, in the latest update.