[FilePreview] Add support for descript.ion files

[octastylos-pseudodipteros]

Summary of the Pull Request

Adds file preview support for descript.ion files

PR Checklist

• Closes: Support .ion-files in Peek #35515
• 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: No need
• New binaries: None
• Documentation updated: No need

Detailed Description of the Pull Request / Additional comments

Adds .ion to txtExt in monaco_languages.json

Validation Steps Performed

Manually tested previewing descript.ion file