[css-sizing-3] Add definition for "intrinsic size"

[fantasai]

We have definitions for a lot of sizing terms, but not "intrinsic size". We should add it.

[Loirooriol]

Isn't this already in https://drafts.csswg.org/css-sizing/#intrinsic-sizes?

[frivoal]

It kind of is, except the term "intrinsic size" itself isn't defined with a <dfn>, causing attempts to cross link to it as a defined term using bikeshed to fail.