Support concurrent rendering in React 18

[mixvar]

• use useSyncExternalStore in internal useSubscription hook (use provided shim so that react peer dependency does not need to be bumped)
• internal useSubscription hook should return value for use in affected hook/component (rather than accessing atom value directly)
• remove FieldDependenciesAtom and extend FieldStateAtom to contain values for branch errors and branch isValidating
• Atom.fuse may need to be extend with optional equality function so that branch erros / branch isValidating atoms can be created without extra re-renders