postcss: Work around postcss-extend-rule incompatibility with :is().

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

web/postcss.config.js

@@ -21,6 +21,7 @@ const config = ({file}) => ({
         postcssSimpleVars({variables: {...container_breakpoints, ...media_breakpoints}}),
         postcssPresetEnv({
             features: {
+                "is-pseudo-class": true, // Needed for postcss-extend-rule
                 "nesting-rules": true,
             },
         }),