babel: Enable shippedProposals.

This has the consequence of turning on loose mode for
@babel/proposal-class-properties, which generates slightly smaller
code.

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

babel.config.js

@@ -7,14 +7,11 @@ module.exports = {
             {
                 corejs: 3,
                 loose: true, // Loose mode for…of loops are 5× faster in Firefox
+                shippedProposals: true,
                 useBuiltIns: "usage",
             },
         ],
         "@babel/typescript",
     ],
-    plugins: [
-        "@babel/proposal-class-properties",
-        ["@babel/plugin-proposal-unicode-property-regex", {useUnicodeFlag: false}],
-    ],
     sourceType: "unambiguous",
 };