Add opus to Peek's allowed file list

[DaveT1991]

Summary of the Pull Request

Adds .opus to Peek's supported audio file extensions so .opus files are routed through the audio previewer instead of being treated as unsupported up front.

PR Checklist

• Closes: .opus support for Peek #42576
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Issue #42576 reports that .opus files are not currently included in Peek's supported audio extension list.

This PR adds .opus to the supported audio file types in AudioPreviewer. That allows Peek to attempt the existing audio preview path for .opus files and use the current fallback behavior if playback is unsupported in the environment.

This is a small, scoped change and does not add any new strings, binaries, or documentation changes.

Validation Steps Performed

• Reviewed the existing previewer flow to confirm audio file extensions are routed through AudioPreviewer
• Verified that .opus was missing from the supported audio extension list
• Confirmed the change is limited to adding .opus to that list

I was not able to run a local build in my current environment because the .NET SDK is not installed.

[daverayment]

Duplicate of #44792.

Also, please note that we do not accept untested PRs.

Please read the contributing guidelines here:

https://github.com/microsoft/PowerToys/blob/main/CONTRIBUTING.md

This is intended to help you and us prevent unnecessary work by agreeing up-front that it is needed.

[daverayment]

Closing as duplicate of #44792