Fix nested bracket type overloading

[sidepelican]

Previously, mockolo fails to generate overloads for nested types like arrays.
Consider the following code, which only produces a single foo function for mock class.

    /// @mockable
    protocol P {
        func foo() -> [String]
        func foo() -> [[String]]
    }

The issue stems from SwiftType.displayName continuing to use a legacy ad-hoc implementation, resulting in both [String] and [[String]] being rendered as the same String.
I'll modify the implementation to properly construct displayName in a structured manner.

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✅ sidepelican
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[uhooi]

LGTM!!

[sidepelican]

Thank you all for your reviews