chore(css): calc-constant renamed to calc-keyword (#2442)

Author: bsmth

features/calc-constants.yml

@@ -1,4 +1,4 @@
-name: calc() constants
-description: The `e`, `pi`, `infinity`, and `NaN` constants are accepted in CSS math functions such as `calc()`.
+name: calc() keywords
+description: The `e`, `pi`, `infinity`, and `NaN` keywords represent well-defined constants accepted in CSS math functions such as `calc()`.
 spec: https://drafts.csswg.org/css-values-4/#calc-keywords
 group: css