Add a HasContent type for withContent usage convenience

[alexrosenfeld10]

Description

See discussion at https://discord.com/channels/832353585802903572/832354150884442122/1167567742350655608

Related Issue

Link to the related issue: See discussion at https://discord.com/channels/832353585802903572/832354150884442122/1167567742350655608

Type of Change (select one and follow subtasks)

• [ ] Documentation (README.md)
• [ ] Maintenance or repo-level work (e.g. linting, build, tests, refactoring, etc.)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
  • [x] I have included test coverage (N/A, type definition only)
• [ ] Breaking change (fix or feature that would cause existing functionality/userland code to not work as expected)
  • [ ] Explain why a breaking change is necessary:
• [ ] This change requires (or is) a documentation update
  • [ ] I have added necessary local documentation (if appropriate)
  • [x] I have added necessary itty.dev documentation (if appropriate) https://github.com/kwhitley/itty.dev/pull/12