feat(btn): full rewrite of button styles

[pdanpdan]

Organize styles so that there is very little need for duplication.
The only styles that do not work with tailwind responsive modifiers are the ones for disabled .btn-link and .btn-ghost.

I have prepared 3 playgrounds that should cover most of the normal usage:

• 5.4.7: https://play.tailwindcss.com/uUitizu33y?layout=preview
• 5.4.8+: https://play.tailwindcss.com/mkA67TyHx8?layout=preview
• full rewrite: https://play.tailwindcss.com/QZaRC8aifn?layout=preview

Problems fixed:

• if buttons (checkbox) are used in .filter they do not show active status
  • also in feat(filter): show all options on keyboard navigation #4427, so it would be better to first merge that one
• aria-disabled="true" is not accounted for
• on all dark themes .btn-neutral .btn-soft cannot be used
  • addressed by introducing an internal variable to set the soft bg color for neutral (--int-soft-bg: var(--color-neutral-content) 80%;)
• .btn-soft, .btn-outline, .btn-dash, .btn-ghost, .btn-link when there is contrast between fg (--btn-fg) and bg (--btn-color) button colors, but there is no contrast in rest state between bg (derivative of --color-base-100 or page bg) and fg (--btn-color)
  • new user facing variable --btn-rest-fg that sets the fg color for buttons in rest state (only for soft, outline, dash, ghost) - it is ignored for .btn-neutral because that color has its own rules
  • .btn-link does not need any mitigation because it uses the same color all the time, so it can already be set by the user
  • in the example --btn-rest-fg is set for the whole theme, but it can be used for individual buttons also
• a checkable button cannot be identified as checked/unchecked when focused
• setting --btn-color and ``--btn-fg` on buttons does not work as expected

close #4430