Abort the build if a 'vendor' entrypoint detected

Previously users were encouraged to create a fake `vendor` entrypoint
which worked in conjunction with `@neutrinojs/chunk` to separate out
third-party dependencies to a vendor chunk.

However with `splitChunks`, manually specifying this entrypoint will
actually result in larger builds - so we must ensure users correctly
update their configuration.

packages/web/index.js

@@ -257,4 +257,13 @@ module.exports = (neutrino, opts = {}) => {
 
       config.output.filename('[name].[chunkhash].js');
     });
+
+  neutrino.on('prerun', () => {
+    if (neutrino.config.entryPoints.has('vendor')) {
+      return Promise.reject(
+        new Error('Vendor chunks are now automatically generated. Remove the manual `vendor` entrypoint.')
+      );
+    }
+    return Promise.resolve();
+  })
 };