[Peek] Terminate Preview Handlers Processes

[davidegiacometti]

Summary of the Pull Request

This is an attempt to release Preview Handlers instantiated by Peek and close the related processe.

⚠️ Note that even if the PR improve the current behavior, the solution doesn't work 100% of times.
I noticed that sometimes the process gets leaked also when Preview Handler is used in Explorer 🤔

PR Checklist

• Closes: [Peek] Preview handler preview is leaking process #40117
• 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

• Ported the same fix applied to CmdPal to ensure the process is terminated gracefully instead of being killed: [CmdPal] Close Gracefully #39589
• Attempt to cleanup Preview Handlers and close the relative process

Validation Steps Performed

Tested manually:

• Preview through some Excel and Word files
• Close Peek window
• Excel and Word processes are closed