Enable support for proposals in the babel runtime transformation

Enable support for proposals in @babel/plugin-transform-runtime because
ESLint and TypeScript will take care of rejecting syntax that is not
enabled by the configured language versions and library definitions, but
proposals that have shipped.
diff --git a/babel.config.js b/babel.config.js
index e274bf9..9251327 100644
--- a/babel.config.js
+++ b/babel.config.js
@@ -65,7 +65,7 @@
   // Options for the @babel/transform-runtime plugin.
   const runtimeOptions = {
     // Use corejs version 3.
-    corejs: { version: 3 },
+    corejs: { version: 3, proposals: true },
     // Use helpers formatted for the target environment.
     useESModules: !CJS && !TEST,
   };