Generalize ParsableVariable and Schemavariables

[Drodt]

Related Issue

Intended Change

This PR moves the ParsableVariable interface to ncore, makes it no longer an Operator and renames AbstractSV to OperatorSV as this interface now handles all schema variables that may be operators.

It also handles the refactorings necessary to adapt to this change. I.e., changing types where necessary, changing how modality schema vars are matched, and some changes to the term builder.

Also fixes the Github workflow.

Type of pull request

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Refactoring (behaviour should not change or only minimally change)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] There are changes to the (Java) code
• [ ] There are changes to the taclet rule base
• [ ] There are changes to the deployment/CI infrastructure (gradle, github, ...)
• [ ] Other:

Ensuring quality

• [x] I made sure that introduced/changed code is well documented (javadoc and inline comments).
• [ ] I made sure that new/changed end-user features are well documented (https://github.com/KeYProject/key-docs).
• [ ] I added new test case(s) for new functionality.
• [ ] I have tested the feature as follows: ...
• [ ] I have checked that runtime performance has not deteriorated.

Additional information and contact(s)

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.