[Monaco] Add support for .srt (subtitle) file previewing

[PesBandi]

Summary of the Pull Request

Adds support for previewing .srt files (Peek and FE dev file preview). They are previewed as plaintext, as suggested by the issue author.

PR Checklist

• [x] Closes: #32769
• [x] Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• [x] Tests: Added/updated and all pass
• [x] Localization: All end user facing strings can be localized
• [x] Dev docs: No need
• [x] New binaries: None
• [x] Documentation updated: No need

Detailed Description of the Pull Request / Additional comments

Adds .srt to txtExt in monaco_languages.json.

Validation Steps Performed

Observing