[css-fonts-5] <meta text-scale> limits

[fantasai]

Following up on #12380 from some internal discussions with @cookiecrook, who noted that a lot of sites think they're scalable with user preferences, but break when they get past the more common large sizes into some of the a11y-enabled extra-large sizes.

Suggestion is that instead of having the two options of "it's fixed sizes for compat" or "it's fully adaptable to the user's font size" it might make sense to specify, as the value of this meta, the largest size the site expects it can comfortably handle. If the user's preferred size is larger than this, the UA clamps it so it never gets a default size larger than it can handle.