Add rhythmic sizing (web-platform-dx#2135)

Co-authored-by: Daniel D. Beck <[email protected]>

Author: 2 people

features/rhythmic-sizing.yml

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+name: Rhythmic sizing
+description: The `line-height-step` CSS property creates consistent vertical rhythm by setting the spacing between body text baselines. It rounds taller lines like headings up to the next multiple of the body text spacing.
+spec: https://drafts.csswg.org/css-rhythm-1/
+group: text
+compat_features:
+  - css.properties.line-height-step

features/rhythmic-sizing.yml.dist

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+# Generated from: rhythmic-sizing.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}
+compat_features:
+  - css.properties.line-height-step