Disallow dialogue stack slot from being set in custom actions & mark builtin slots

[ancalita]

Proposed changes:

• Implements ATO-1624

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[twerkmeister]

Code looks good to me for now. One thing we have to be careful with: Some built-in slots should be writable. E.g. we have the return_value slot that is there as convenience for setting it in custom actions so that you don't need a new slot for one-off results. Maybe instead of is_builtin it should be is_user_writable. We could have a separate list where we list the writable ones instead of reusing the existing slot lists. Let's keep it in mind for later.

[ancalita]

Some built-in slots should be writable.

I guess it depends on how the new property marker is_builtin will be used in the future. I'll document somewhere that this property captures both writable and read-only builtin slots (perhaps in the slot module).

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

5 Bugs
0 Vulnerabilities
3 Security Hotspots
52 Code Smells

0.0% Coverage
0.9% Duplication

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