[css-conditional-5] Use the generic <boolean-expr>

[cdoublev]

As a consequence of 5f19369, <boolean-condition> is extracted by w3c/reffy and exposed to w3c/webref users.

However, <boolean-condition> was intended to be defined generically, as noted under its definition, and this has been done with <boolean-expr> in 4fd47d8 in CSS Values 5.

Therefore this PR replaces <boolean-condition> with <boolean-expr[ <if-test> ]> in the prelude of @when and @else. The leaves of the boolean condition/expresion are consequently different:

- media( [ <mf-plain> | <mf-boolean> | <mf-range> ] ) + media( <media-feature> | <media-condition> ) | - supports( <declaration> ) + supports( [ <ident> : <declaration-value> ] | <supports-condition> ) | + style( <style-query> )

I assume these differences should not exist and will be fixed later if required, and that accepting style() queries is ok.