Convert extension methods to instance methods in the next breaking release

[nex3]

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https://github.com/dart-lang/source_span/pull/49 adds some extension methods on SourceSpan. These are being added as extensions to avoid breaking downstream classes that implement SourceSpan without including SourceSpanMixin, but they would ideally be instance methods whose implementation would be provided with the mixin but could also be overridden by downstream users. The next breaking change should fix this.