Allow manual changes to generated stub files

[lars-reimann]

Is your feature request related to a problem? Please describe

Some modifications to stub files should be allowed, such as adding more constraints, tighter datatypes, or schema effects (basically everything that does not affect the Python code). Piping these through the API editor does not make much sense since the user would need to write DSL code in there to get the same generality but we cannot offer ideal tool support.

Desired solution

Take existing stub files as input when creating the new stubs (if available) and try to merge the generated stubs with them. Report any conflicts and let the user resolve them.

Additional context (optional)

Changes to Python code could also be allowed, however, currently only interface changes are supported instead of implementation changes, which heavily limits usefulness.

Prerequisite: #327 (to parse the existing stubs)