Add Custom Media Queries (#3325)

Co-authored-by: Patrick Brosset <patrickbrosset@gmail.com>
Co-authored-by: Daniel D. Beck <daniel@ddbeck.com>

Author: 3 people

features/custom-media-queries.yml

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+name: Custom media queries
+description: The `@custom-media` CSS at-rule names a media query, which you can reuse in multiple `@media` rules.
+spec: https://drafts.csswg.org/mediaqueries-5/#custom-mq
+group: media-queries
+# Not in BCD yet, but expecting something like:
+# - css.at-rules.custom-media
+# - api.CSSCustomMediaRule
+# - api.CSSCustomMediaRule.name
+# - api.CSSCustomMediaRule.query

features/custom-media-queries.yml.dist

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+# Generated from: custom-media-queries.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}