Improve TranslationContext type narrowing using a tagged union

[bryanforbes]

Summary

This fixes type narrowing for TranslationContext. Previously, conditions other than command_name weren't narrowing correctly.

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [x] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

I'm not really big on defining a separate object for this just for the types, especially one named TranslationContextObject.