Fix compiler bug with rank 2 types in mapAccuml

[CharlonTank]

This PR fixes a compiler bug that occurs when using mapAccuml with explicit type annotations. The bug manifested as a compiler crash with the message "x [rank = 2]" followed by "thread blocked indefinitely in an MVar operation".

The issue was that the compiler was incorrectly treating rank 2 type variables as invalid during generalization. These rank 2 variables are actually valid and necessary for higher-order functions like mapAccuml.

The fix modifies the isGeneric function to specifically allow rank 2 variables, which enables proper type inference for mapAccuml and similar functions.

Test case that previously crashed but now works properly:

func =
    List.Extra.mapAccuml
        (\acc x ->
            let
                newAcc = acc + 1
                result = x + acc
            in
            ( newAcc, result )
        )
        0
        [1, 2, 3]

Unfortunately, it (hot)fixes the problem but there surely are some side-effects

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