fix(developer): readonly groups should never emit output

Fixes #6799.

For KeymanWeb compiled keyboards, readonly groups should never emit any
output. Rules in these groups permitted only to:
  * `use` other readonly groups
  * `call` external functions (which MUST NOT emit output)
  * `set`, `reset` or `save` store values

The base kmx compiler does emit output for rules in readonly groups.
Thus, the KeymanWeb compiler, when it transpiles from the generated
.kmx, needs to omit output for any readonly group, while avoiding any
other potential change in behaviour.