[mediaqueries-5] Allow `@custom-media` inside `@supports` or add `@custom-supports`

[argyleink]

https://drafts.csswg.org/mediaqueries-5/#at-ruledef-custom-media

Consider the following styles where it feels wrong to use a custom-media inside supports right?

@custom-media --motionOK (prefers-reduced-motion: no-preference);
@custom-media --scrollTimeline (animation-timeline: works);

@media (--motionOK) { ... }
@supports (--scrollTimeline) { ... }

adding @custom-supports to the spec would allow proper type checking? making this the example instead:

@custom-media --motionOK (prefers-reduced-motion: no-preference);
@custom-supports --scrollTimeline (animation-timeline: works);

@media (--motionOK) { ... }
@supports (--scrollTimeline) { ... }

thoughts?

I very much love using @custom-media btw, very rad.

[dbaron]

Today the CSSWG resolved to accept Tab's when-else draft proposal into Conditional 4, which has media() and supports() functions. Perhaps @custom-media should instead be something that goes in @when, and then there could be only one? Alternatively, maybe there should be a way of doing @supports queries inside @media rules, or vice-versa, or both?