Remove unnecessary dependencies so we no longer need npm install --legacy-peer-deps

[mturley]

Resolves #134.

We have two direct dependencies which do not have new releases that support React 17, but which we no longer need:

• enzyme-adapter-react-16 which is orphaned and no longer actually used in tests since @wojtekmaj/enzyme-adapter-react-17 was added
  • This PR simply removes the unused dependency.
• react-router-last-location, which was added in Focus new content on route changes #57 as part of a workaround for SPA Accessibility - focus not reset when route changed remix-run/react-router#5210.
  • This PR removes the dependency by changing useA11yRouteChange to a more simple implementation based on useLocation. The focus handler will trigger only if the pathname property changes (not if only the URL query string changes). This is based on the workaround found in this comment.
  • As part of updating useA11yRouteChange, this PR also resolves a regression I described in chore(upgrade): upgrade deps, fix storybook #103 (comment) related to the isAsync property that stuck around on the route configs after async route support was removed. See that comment for more details.

With those two adjustments, all of our production dependencies support React 17 and npm install no longer fails when used without the --legacy-peer-deps flag. There are still a lot of peer dependency warnings related to Storybook's dependencies that are not compatible with React 17. I tried upgrading to the latest Storybook (and also to their prerelease versions) and that didn't resolve those warnings, so I left Storybook alone in this PR and limited its scope to the errors that were causing npm install to fail.

It seems these warnings may eventually go away if we upgrade to a future version of Storybook which only has dependencies that support React 17. Links I was looking at as part of figuring this out:

• Fix NPM7 peer dependencies storybookjs/storybook#14119
• Upgrade @reach/router to react-router@6 storybookjs/storybook#14619

I opened #151 to track that after this is merged.