feat: add utility for menu styles

[jzempel]

Description

With the new menuStyles utility, menus in Garden are now clearly:

1. A positioning wrapper (facilitated via Popper)
2. A styled menu child

Features

• New menuStyles utility added to react-theming and shared between react-datepickers and react-dropdowns
• Ability to target menu wrappers with themed component custom styling

Fixes

• Added visibility controls (via useEffect with timeouts) to restore fade-out animation for both datepicker and dropdown menus –  providing subtle, important feedback that a user's action was effective
• Implemented CSS that cut out unnecessary "div soup" in dropdown menus.
  • Hiding arrow border during first phase of animation got rid of StyledMenuAnimation div.
  • Overriding menu positioning to static allows arrow to display beyond overflow-y and got rid of StyledMaxHeightWrapper div.
• Restored semantic <ul> - <li> menu structure in dropdowns.

Other

• Document no-JSX expectation for view components. This cleans up code clutter and, in this PR, improved bundle sizing.

Detail

Demo site is pre-published for review. Notable links include:

• theming with knob controls to demonstrate the new menuStyles functionality
• datepickers updated to ensure the calendar dismiss fades out as expected
• dropdowns updated to ensure the menu dismiss fades out as expected

Merge of this PR will be paired with #651 to publish react-components v8.1.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage decreased (-0.1%) to 98.067% when pulling db1cf6c on jzempel/menu-styles into 893194b on master.

[austingreendev]

Left some comments, but overall this looks good.

We will also need to bump the peerDependency versions for react-theming to ^8.1.0 in all packages. I can do that in #651

[austingreendev]

So this was the trick when I first moved over to transform: translate for the animation. Good find.

[ginnywood]

This is awesome, the animation looks great. Noticed a couple weird things:

1. In the Tree Menu usage, if I select an item from the nested menu, I see a flash of the parent menu as it fades out. https://share.getcloudapp.com/rRu67RyB
2. In the Multiselect, selecting an item adds it as a tag and text. This happens on garden.zendesk.com too, so it’s outside the scope of this branch, but I thought I’d flag it. https://share.getcloudapp.com/JruW6BX9

[jzempel]

Nice finds @ginnywood. I'm going to add an issue for us to treat these as follow-on bugs 🐛