[css-contain-3] Define the block contents of @container with <rule-list>

[cdoublev]

<block-contents> seems confusing to me because declarations are allowed only when @container is nested (at any level) in a style rule.

It is also inconsistent with other conditional rules (@media and @supports), whose block contents is defined with <rule-list>.

All conditional group rules are defined to take a <rule-list> in their block

https://drafts.csswg.org/css-conditional-3/#contents-of

[tabatkins]

Agreed, it should be consistent with how we define @media/etc - <rule-list> in the official grammar, and let Nesting override that when nested (it specifies that the <rule-list> turns into a <block-contents> in this case).