Refactor dropbutton CSS

Problem/Motivation

The dropbutton CSS is super crufty, overcomplicated, and hard to manage. It uses JS to flip the dropdown when overflowing (this can now be done with CSS).

Proposed resolution

• Much smaller and simpler CSS.
• The dropdown is positioned with native CSS instead of JavaScript. migration/js/dropbutton.js is removed. CSS anchor positioning now places the dropdown, and the browser flips it above the button when there isn't room below.
• The dropdown is no longer cut off by its container. Because it's position: fixed and anchored, overflow: hidden on a parent (table cells, narrow layouts) can't clip it.
• Variables are scoped to the component. Dropbutton variables now live on .dropbutton instead of :root, and the unused ones are removed.
• Fewer !importants. There are 2 now instead of 15, and both have a comment saying why they're there.
• Simpler high-contrast (forced colors) mask colored with currentColor, plus a single buttonText override in forced-colors mode. This replaces the old separate CSS-border chevron, which had its own rotation and margin math for each
• Better no-JS support. Without JavaScript (@media (scripting: none)), the dropdown opens on hover or keyboard focus.
• Long menus scroll.
• The CSS loads only when needed. dropbutton global-styling library to the dropbutton library, so pages without a dropbutton no longer load it.

Testing instructions

This MR simplifies and improves the dropbutton. It removes over half of the CSS, makes sure it only loads when needed, and also deletes JavaScript. It replaces the JS functionality with pure CSS. It has fallback support for non-JS environments, and also improves accessibility.

To test:

1. Switch to the new branch and clear cache. Verify dropbuttons work as expected.
2. For a page that shows all variations of dropbuttons, install the Admin Theming Tools module, and install the dropbutton submodule.
3. Verify that dropbutton looks great and works within the Views UI
4. Verify the block UI and field UI’s dropbuttons work as expected.
5. Test in Chromium, Safari, and Firefox
6. Test in RTL
7. Test in both dark and light modes.

Notes

• Notice that if a dropbutton would open and overflow the bottom of the page, it will open to the top.
• Note that the styles of the dropbutton are not exactly the same. The padding around the exposed link has been standardized with the items in the dropdown. The size of the extrasmall variant has been increased slightly to ensure the trigger button meets 24px, to comply with the WCAG 2.2 AA tap target criteria.