Draft: a11y: Add text size slider

Replace Large Text toggle with a continuous slider.

Fixes #1648

Implemented like a suggestion by Allan in #1648 (comment 1769435).

This is in no way final, and if it gets purged, fine with me. I made this just to test what it would look like and how it would work, and for others to try it out and perhaps help the discussion along.

Thoughts

• The updating is quite slow, even on a fast PC. This means something like Teams/Design/settings-mockups#61 (comment 1935264) is probably required. With 0.05 steps, it looks okay.
• There are quite a few steps that don't seem to really increase size, but just the spacing? You can see this by stepping the scale with the arrow keys. This can be improved by restricting the minimum change to something like 0.05?
• Weirdly enough, Firefox only scales when you hit 1.25. That is exactly the value for the old Large Text toggle. It looks like that is hardcoded in some apps then? I did not know that.

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