Similar to Bun's --test-only, it would only run tests marked as only. This is useful when only one / a few test cases are giving trouble, and the user would like to debug them. Instead of having to step through the breakpoints of all the other tests, or commenting the tests, the user can mark them with only and run only those.

• test() -> test_only()
• define() -> define_only()
  • Subsuites and tests automatically marked as only
• test_*() -> test_*__only()
  • Append keyword, so it's easier to remove, even if slightly less readable
  • Specifically using __only, so suffix can't be mistaken for a legitimate test name
• Press Shift+F7 to run only
• Press Ctrl+Shift+F7 to debug only