feat: Add support for visionOS

[bmueller]

New Pull Request Checklist

• [x ] I am not disclosing a vulnerability.
• [x ] I am creating this PR in reference to an issue.

Issue Description

Adds visionOS support.

Closes: #1761

Approach

Just added TARGET_OS_VISION in a bunch of places. Also updated the readme to mention visionOS alongside the other supported platforms.

TODOs before merging

• Add tests
• [x ] Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[dplewis]

@bmueller Thank you for the PR, everything looks good so far! Waiting for #1763 to be merged to fix the CI and to prevent any merge conflicts with this PR.

The next steps would be:

• Adding .visionOS(.v1) to Package.swift
• Adding Badge to README
• Adding a VisionOS starter project and building it in the Rakefile to run in the CI to ensure these changes work.
• Possibly add PF_VISION_UNAVAILABLE and PF_VISION_UNAVAILABLE_WARNING as needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.24%. Comparing base (dd05d41) to head (492bba7).
Report is 3 commits behind head on master.

❗ Current head 492bba7 differs from pull request most recent head 21457a9. Consider uploading reports for the commit 21457a9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1762   +/-   ##
=======================================
  Coverage   64.24%   64.24%           
=======================================
  Files         201      201           
  Lines       23233    23233           
=======================================
+ Hits        14926    14927    +1     
+ Misses       8307     8306    -1     

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[mtrezza]

@bmueller Would you want to add a visionOS job to the CI as well, so we can make sure it compiles?