Type ASTProcessor targets and ASTReference nodes with protocols

[FrancescoPezzella]

Generic code typed its node parameters as ASTNode, but only the Clang adapters subclass ASTNode. The Python RST and CST adapters and the tree-sitter adapter are duck-typed (ADR 03, ADR 06). They already worked at runtime but failed type checking against ASTProcessor and ASTReference. This PR types those APIs with the existing protocols instead, so any adapter that satisfies them type-checks.

No runtime behaviour changes: only type annotations, one TODO and test fixture annotations.

Refers to #269, partially fixes some of the issues
Relates to #149

[FrancescoPezzella]

Ci/Cd is failing even though more lint errors were solved, 6460 > 6352 (+100 issues solved). Reason for that is that solving one issue, relegated the error somewhere else, exposing another issue that was hidden by the former issue solved. In this way, I would be solving all Type errors for the rest of the days, so to keep the scope of this PR smaller, I will fix some unrelated Type errors to make Ci/Cd pass.

ruff: 2219 issues in 56 kinds, budget 2274
pyright: 6352 issues in 32 kinds, budget 6460

These kinds of issues exceed their budget:
  pyright reportUnknownArgumentType: 1175 issues, budget 1121 (+54)
  pyright reportUnknownVariableType: 895 issues, budget 891 (+4)

[pjljvandelaar]

Some questions raised.
But further looks good.