Fix compiler bug with rank 2 types in mapAccuml

[CharlonTank]

This PR fixes a compiler bug that occurred when using mapAccuml with explicit type annotations. The bug manifested as a compiler crash with the message:

thread blocked indefinitely in an MVar operation

The issue was that the compiler was incorrectly rejecting rank-2 type variables during generalization. These variables are in fact valid and required for higher-order functions like mapAccuml.

The fix updates the isGeneric function to explicitly allow rank-2 variables, enabling proper type inference for mapAccuml and similar functions.

Example of a test case that previously crashed but now works correctly:

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

⚠️ Note: This change is effectively a hotfix. While it resolves the immediate problem, it may introduce side effects elsewhere.

[github-actions]

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