[css-mixins-1][css-extensions-1] Improve grammars of at-rule preludes

[cdoublev]

Would it be possible to add contextual productions for the value definitions of the prelude of @function, @mixin, @env, @custom-media, @custom-selector?

For example:

- <@function> = @function <function-token> <function-parameter>#? )
-   [ returns <css-type> ]?
- {
-   <declaration-rule-list>
- }
+ <@function> = @function <custom-function-definition> { <declaration-rule-list > }
+ <custom-function-definition> = <function-token> <function-parameter>#? ) [ returns <css-type> ]?

The main reason is simply convenience in implementing declarative grammar validation (eg. an invalid @function parameter type).

I think this can also help to clarify them and to understand the terminology defined in prose.

Similarly, would it be possible to add a production for the value definition of a custom selector name?

- <custom-selector> = <custom-arg>? : <extension-name> [ ( <custom-arg>+#? ) ]?
+ <custom-selector> = <custom-arg>? <custom-selector-name> [ ( <custom-arg>+#? ) ]?
+ <custom-selector-name> = : <extension-name>

Happy to submit these changes is that sounds good to the editors of the related specs.