[css-inline-3] Do not expand `text-box` value definition?

[cdoublev]

I see two drawbacks to this:

Issue 13: This propdef table is fully expanded to evaluate and adjust the shorthand’s value space. It simplifies down to <text-box-trim> || <text-box-edge> minus the none and auto keywords.

First, this is error prone and slightly more maintenance work when you add a new longhand value.

Second, depending on the CSS parser, this may require more implementation work than making none and auto invalid.

Nit: the fully expanded value definition would be normal | [<'text-box-trim'> || <'text-box-edge'>].

[fantasai]

This is just a temporary editorial measure to facilitate review. :) Agree we should use the standard notation eventually.