Use longer-form blank for dummy type name

[vexx32]

PR Summary

Adds the type ____ so that type blanks look a little more in line with current recommended format for text blanks (now using [____] rather than [__] to match '____').

Changes

• Added ____ to DummyTypes.ps1
• Changed all existing koans to use [____] instead of [__]
• Retained [__] dummy type so old koans don't throw a fit and give Pester errors.

Checklist

• Pull Request has a meaningful title.
• Summarised changes.
• Pull Request is ready to merge & is not WIP.
• Added tests / only testable interactively.
  • Make sure you add a new test if old tests do not effectively test the code changed.
• Added documentation / opened issue to track adding documentation at a later date.