[mediaqueries-5][css-conditional-5] `@custom-media` and `@supports-condition`

We have [`@custom-media`](https://drafts.csswg.org/mediaqueries-5/#custom-mq) which defines a "custom media query" and [`@supports-condition`](https://drafts.csswg.org/css-conditional-5/#supports-condition-rule) which defines a "named supports condition." Should we use more consistent spellings and terminology for these two things?
