[Feature Request]: Add new `get_messages` method.

[llrs-roche]

Feature description

The qenv article exposes the internal representation of objects like qenv@warnings and qenv@messages.

It is usually not recommended to expose the internal object slots to users: this gives more freedom to developers to later change internal representations or even classes without users experimenting friction.
There is a getter for qenv@warnings: get_warnings() but none for messages. An additional get_messages() method could avoid this situation.

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[m7pr]

@gogonzo I think this is vital functionality, as there is no way to work with qenv@messages right now. Should we add this function during #216 ?

[gogonzo]

@m7pr this can be addressed separately. Let's not complicate already complex PR 👍