`prefer_const_constructors_in_immutables` support for extension types

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More discussion: #53351.

The question is whether we want to treat extension types as effectively annotated @immutable (since they essentially are) and lint all non const constructors on extension types.

/cc @goderbauer @a14n @eernstg

[pq]

As per the discussion in #53351, there might be good reasons for non-const constructors in extension types so it's reasonable to allow annotating them with @immutable and get the benefits of this lint.

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(linked from wrong PR)

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Fixed w/ 532e467.