Declarative button actions edits

[dvdherron]

Closes #331

• Cleans up shape of inline polyfill
• removes timing benefit for custom commands
• consistent event delegation
• make certain instructions more actionable (“It must have a unique id.” → “If it doesn’t have a unique id , add one”)
• Consistent command registry approach
• Rename to make it clear that this is about custom commands only (This was updated in a separate PR but I think it's along the same lines? Open to simplifying).

[dvdherron]

@LeaVerou I know there's mention of splitting up/merging this guide. Wanted to do some cleanup beforehand in case we decide to keep this use case for now.