Twenty Twenty-One: Bring back PostCSS config.

Twenty Twenty-One uses PostCSS to convert the custom properties for Internet Explorer. Without a config file, the postcss command does nothing, causing issues in IE. This brings back the config from GitHub, which replaces all custom properties with the default values.

Props t-p, poena, ryelle.
Merges [49800] to the 5.6 branch.
Fixes #52040.

git-svn-id: https://develop.svn.wordpress.org/branches/5.6@49824 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-content/themes/twentytwentyone/postcss.config.js

@@ -0,0 +1,12 @@
+module.exports = {
+	plugins: [
+		require('postcss-nested'),
+		require('postcss-css-variables')({
+			preserve: false,
+			preserveAtRulesOrder: true
+		}),
+		require('postcss-calc')({
+			precision: 0
+		})
+	]
+};