[RegPreview] Init with header and add NEW button

[htcfreek]

Summary of the Pull Request

This PR improves Registry preview to allow pasting registry keys and values without writing the file header manually.

This is done by adding the following changes:

1. Init the editor with only the registry file header if no file is opened.
2. Adding a new button that resets the app to the "init without file" state.

PR Checklist

• Closes: Registry Preview: allow pasting a registry key without saving it to a file #36872
• Communication: I've discussed this with core contributors already. If 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

Validation Steps Performed

Run local build and validate the behavior.