Describe your changes

• Fix unescapableCommand triggering on every arrow down press within the Snippet (e.g. in the a central language block, pressing down would cause a blank space despite not being at the end of the Snippet)
• Run unescapableCommand checks on up arrows too (e.g. you should be able to move above a snippet with the cursors)
• Allow External Plugins commands to override pre-definied editor commands
  • Use the ability to override editor commands to swallow commands that are nonsensical for Snippets, and cause breaking changes to happen (e.g. inserting a line-rule in the middle of the Snippet)