(test): ensure a regenatorRuntime import is added

- when using transform-runtime with helpers: false, it should import
  from @babel/runtime/regenerator
- either this plugin configured like this or a direct import of
  regenerator in one's own library code is the only way to get it to
  be imported
  - just using a generator will make regenerator code added, but
    without any import (leading to a ReferenceError)
  - TODO: make it easier or automatic to use generators
    - useBuiltIns of preset-env?

test/integration/fixtures/build-withBabel/.babelrc.js

@@ -1,5 +1,6 @@
 module.exports = {
   plugins: [
-    'styled-components'
+    'styled-components',
+   ['@babel/plugin-transform-runtime', { helpers: false }],
   ]
 }

test/integration/fixtures/build-withBabel/src/generator.ts

@@ -0,0 +1,3 @@
+export function* testGenerator() {
+  return yield 'blah';
+}

test/integration/fixtures/build-withBabel/src/index.ts

@@ -1,5 +1,7 @@
 import './styled.tsx';
 
+export { testGenerator } from './generator';
+
 export const sum = (a: number, b: number) => {
   if ('development' === process.env.NODE_ENV) {
     console.log('fuck');

test/integration/tsdx-build-withBabel.test.js

@@ -32,6 +32,13 @@ describe('integration :: tsdx build :: .babelrc.js', () => {
     expect(output.code).toBe(1);
   });
 
+  it('should add an import of regeneratorRuntime', () => {
+    const output = shell.grep(/@babel\/runtime\/regenerator/, [
+      'dist/build-withbabel.*.js',
+    ]);
+    expect(output.code).toBe(0);
+  });
+
   it('should compile files into a dist directory', () => {
     expect(shell.test('-f', 'dist/index.js')).toBeTruthy();
     expect(