ParamType and other typing improvements

[kdeldycke]

WIP PR to track and study the unmerged changes from @AndreasBackx that were left in the stale typing/paramtype. See the discussion at #3329 for context.

I was able to salvage and rebase on top of stable the majority of the initial changes like:

• making ParamType a generic ABC and introducing ParamTypeInfoDict
• adding ParamTypeInfoDict
• updating FuncParamType to a generic
• narrowing of convert() return types
• CompositeParamType generic with abstract arity

Stuff that were made obsolete from PRs merged upstream since the last commit in 2024:

• BoolParamType: rewritten in Fix reconciliation of envvar with default, flag_value and type parameters for flag options #2956
• Choice normalization: already part of Expand Choice token normalization + make generic #2796
• File docstring: already cleaned up in Break up arguments section #2586

Other stuff that I skipped as too strict:

• _compat.py t.Any to t.AnyStr
• File generic with t.AnyStr
• _is_file_like with t.AnyStr

I also extracted from the original typing/paramtype the refactor of convert_type which is not strictly related to typing improvements and live in its own PR for later evaluation at: #3372

Related to #2805