Transpile @vector-im/compound-web with babel (#25906)

webpack.config.js

@@ -271,6 +271,12 @@ module.exports = (env, argv) => {
                         if (f.startsWith(reactSdkSrcDir)) return true;
                         if (f.startsWith(jsSdkSrcDir)) return true;
 
+                        // Some of the syntax in this package is not understood by
+                        // either webpack or our babel setup.
+                        // When we do get to upgrade our current setup, this should
+                        // probably be removed.
+                        if (f.includes("@vector-im/compound-web")) return true;
+
                         // but we can't run all of our dependencies through babel (many of them still
                         // use module.exports which breaks if babel injects an 'include' for its
                         // polyfills: probably fixable but babeling all our dependencies is probably