sem: varargs now have and conform to a specification

[saem]

Summary

• created a battery of tests specifying vararg parameters and to a lesser extent the pragma
• updated the implementation for varargs argument and parameter handling to conform to the aforementioned tests
• updated the docs to describe the latest behaviour and indicate future direction

Details

• info that couldn't fit into the summary
• relevant details; tricky parts
• anything else

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Notes for Reviewers

• the plan for this PR is to merge in tests and the rework of sigmatch in one go
  • reason for doing both simultaneously is because the spec itself needs to be vetted through implementing
• created initial as a non-draft PR to ensure the existing tests/knownIssues are at least green in CI
• rough plan, in case you have feedback on the approach:
  • get initial tests in place
  • add docs to tests clarifying the intention from the notes remaining in tvarargs
  • initial revision of docs (manual.rst) to indicate new behaviour and future direction
  • rewrite sigmatch.matchesAux which contains the parameter handling
    - [ ] any related updates/changes
  • write a proper PR title and message

[saem]

One of the things blocking this PR is that the core matchesAux loop needs to work on a per formal parameter basis instead of per argument as it is now. This allows for the progress of matching against formals to be controlled more precisely.

As part of that I believe one of the things that needs to happen is to move default parameter handling/setting to happen in matchesAux as well, instead of in matches, and possibly matchesGenericParams.