feat: add observable queue

[amondnet]

Describe the changes proposed in this Pull Request.

Fixes https://github.com/mobxjs/mobx.dart/issues/887

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Pull Request Checklist

• [ ] If the changes are being made to code, ensure the version in pubspec.yaml is updated.
• [ ] Increment the major/minor/patch/patch-count, depending on the complexity of change
• [ ] Add the necessary unit tests to ensure the coverage does not drop
• [ ] Update the Changelog to include all changes made in this PR, organized by version
• [ ] Run the melos run set_version command from the root directory
• [ ] Include the necessary reviewers for the PR
• [ ] Update the docs if there are any API changes or additions to functionality