[css-containment] Splitting the "sizing" part from "layout" containment

Ojan recently brought up with me that, for the most part, the
containment effects are fairly mild.  It's usually totally fine to
apply contain:strict liberally across your page, except for one
problem - the sizing part of contain:layout totally messes things up.

Now, the size containment is still *useful* - it keeps you honest when
you are in a situation where you can do fixed sizing, preventing you
from accidentally styling something to have non-fixed size.  It's also
useful for userland implementations of "container queries" to prevent
loops, especially if we can activate it *solely* for the inline axis
(letting the block axis still auto-size based on children).

Ojan suggested splitting it out, though, so we end up with *four*
containment types.  Then "strict" would only be equal to "layout style
paint", not "layout style paint size", and would usually be safe to
throw around your page.  This should be a nice usability win, making
it easy to apply perf enhancements via 'contain' without risking
blowing up your page.

In other words, we'd change it to:

contain: [ strict | [ layout || paint || style ] ] || [ size | inline-size ]

and "contain:strict" would be equivalent to "contain: layout paint
style".  To get the *full* effect (with the easily-destructive size
containment) you'd have to write "contain: strict size;" or similar.

Alternately, we could remove the sizing part entirely from 'contain',
and move it to another property entirely, perhaps in the Sizing spec.
This keeps "contain:strict" having the naive meaning of "all the
containment", and perhaps gives a more usable/logical interface for
the size stuff.

Thoughts?

~TJ