[red-knot] Add `Type::TypeVar` variant (#17102)
This adds a new `Type` variant for holding an instance of a typevar
inside of a generic function or class. We don't handle specializing the
typevars yet, but this should implement most of the typing rules for
inside the generic function/class, where we don't know yet which
specific type the typevar will be specialized to.
This PR does _not_ yet handle the constraint that multiple occurrences
of the typevar must be specialized to the _same_ time. (There is an
existing test case for this in `generics/functions.md` which is still
marked as TODO.)
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>
