refactor!: Make query() result an Iterable

[filiph]

Description

Avoid hard-to-detect bugs by disallowing people to touch the list that they get from query().

Checklist

• [x] I have followed the Contributor Guide when preparing my PR.
• [ ] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [ ] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [x] Yes, this PR is a breaking change.
• [ ] No, this PR is not a breaking change.

Related Issues

[filiph]

As discussed, this PR includes a dart doc that might help developers when using query(). This change assumes the List --> Iterable change in OrderedSet.

[spydon]

ordered_set 6.0.0 is now released so this PR can be updated to depend on that :)

[filiph]

Done.