Add items-baseline-last utility

[hbendev]

This PR adds the items-first-baseline and items-last-baseline utility classes that can control baseline-position for the align-items property.

Browser support:

• MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/align-items#browser_compatibility
• Can I use first baseline: https://caniuse.com/?search=align%20items%3A%20first%20baseline
• Can I use last baseline: https://caniuse.com/?search=align%20items%3A%20last%20baseline

The feature were requested multiple times:

• Support additional align-items #13518
• Add support for items-last-baseline #12406
• Add items-last-baseline #11623
• First/last baseline utilities #4855

A special plugin in tailwind.config.js can be used as an existing workaround:

plugin(function extendItemsBaselineUtilities({ addUtilities }) {
  // https://developer.mozilla.org/en-US/docs/Web/CSS/align-items#start
  // https://developer.mozilla.org/en-US/docs/Web/CSS/align-items#end
  addUtilities({
    ".items-first-baseline": { "align-items": "first baseline" },
    ".items-last-baseline": { "align-items": "last baseline" },
  });
});

V3 counterpart PR: #13887

[philipp-spiess]

Hey! Thanks for your contribution. We want to land this in Tailwind CSS v4.1 so I've gone ahead and updated the PR and put it behind a feature flag for now. 👍

I did make some changes to call it items-baseline-last and removed the start-equivalent since it's the default for items-baseline anyways. I added last as a suffix for consistency with our plans with the safe-alignment APIs but note that we might change the name before the actual release.