SuperTextField example is broken

[britannio]

The regression was introduced in #227 and can either be fixed by making the following list non const

super_editor/super_editor/example/lib/demos/supertextfield/_interactive_demo.dart

Line 17 in 1ca4e21

spans: AttributedSpans(attributions: const [

or by storing a mutable copy of the list provided to AttributedSpans so that it can be sorted.

super_editor/lib/src/infrastructure/attributed_spans.dart

Line 45 in 65fd55f

_sortAttributions();