WIBNI: DM demo system: conditional display and execute of lines

### Describe the issue you are having
It would be nice if Ride's DM demo system allowed me to:
- Execute a script line without displaying it
- Display a script line without executing it

This probably entails having an "escape convention" to identify special lines in the script. User command ]demo uses a leading '&' as an escape character. For example:
```
&⍎ 'cmpx'⎕cy'dfns' ⍝ behind-the-scenes copy
&⎕ This comment placed in the session but not executed
```

Start by reading the DM demo system and the ]demo user command, then trace how script lines are displayed and executed. Done means the escape convention supports executing a line without displaying it and displaying a line without executing it, including the examples in the issue.

Written by the indexing model from the issue text.