postcss: Add postcss-calc plugin.

As foreshadowed by commit d0634181b5
(#13115).

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

package.json

@@ -43,6 +43,7 @@
     "moment": "^2.24.0",
     "moment-timezone": "^0.5.25",
     "plotly.js": "^1.48.1",
+    "postcss-calc": "^7.0.1",
     "postcss-extend-rule": "^3.0.0",
     "postcss-loader": "^3.0.0",
     "postcss-nested": "^4.1.2",

postcss.config.js

@@ -5,6 +5,7 @@ module.exports = ({ file, options }) => ({
         "postcss-nested": {},
         "postcss-extend-rule": {},
         "postcss-simple-vars": {},
+        "postcss-calc": {},
         autoprefixer: {},
         cssnano: options.env === "production" ? {} : false,
     },