Add option for always visible floating labels

[mdo]

Fixes #36026.

Needs another pass to see if there's another selector I can use instead of a modifier class, but for now I think an explicit opt-in for this is the best route.

[louismaximepiton]

Reviewing this PR months after the last one.

The context seems clearer to me right now. I took the liberty to add a commit to handle new things mentionned in the issue. Feel free to debate or revert 72ee1af.

My proposal here is to display the placeholder once the input is focused and try to better the solution to make it applicable on each scale.

[julien-deramond]

I haven't reviewed the PR but I did the following to help moving forward with it:

• Rebased and resolved the conflicts. @mdo and @louismaximepiton I’ll let you both double-check the rendering as resolving the conflicts while keeping both your versions wasn’t easy, especially since floating labels had evolved in the main branch in the meantime 😬.
• Changed the IDs to avoid docs compilation issues (8915efa)
• Added the newly created mixins to the floating labels docs (8446581)

[mdo]

Modernized for v6 in #42833 (tokens, label-first :has() markup, no separate Sass mixins). Closing this in favor of that PR.