Updated ReactionSpec.equals type to include supported string shorthands

[jskupsik]

Hoist P/R Checklist

Pull request authors: Review and check off the below. Items that do not apply can also be
checked off to indicate they have been considered. If unclear if a step is relevant, please leave
unchecked and note in comments.

• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required.
• Created Toolbox branch / PR, or determined not required.

If your change is still a WIP, please use the "Create draft pull request" option in the split
button below to indicate it is not ready yet for a final review.

Pull request reviewers: when merging this P/R, please consider using a squash commit to
collapse multiple intermediate commits into a single commit representing the overall feature
change. This helps keep the commit log clean and easy to scan across releases. PRs containing a
single commit should be rebased when possible.

[jskupsik]

I noticed that we fully support string shorthands for a couple default mobx comparer types, ie { equals: 'shallow' }, but the ReactionSpec type definition did not help indicate this fact.

ReactionSpec is now a type because the interface+extends declaration resulted in a typescript error. Types and interfaces are very similar - this should not cause any issues.

[lbwexler]

👍🏻