refactor(tests): Update bootstrap.js to better support generator chunks (#7171)

Refactor bootstrap.js and bootstrap_helper.js to be able to deal
with generator chunks.  In particular for each chunk, specify:

- The goog.module ID to goog.require() in uncompressed mode.
- The script filename to load in compressed mode.
- Where the chunk's UMD wrapper will save the export object when
  loaded as a script.
- What global variable the chunk's export object should be saved in
  (if desired).
- Any individual named exports to destructure to global variables.

This allows the bootstrap scripts to be slightly simpler while
also being more flexible.

tests/playground.html

@@ -8,7 +8,7 @@
      compressed when it is being hosted or on Internet Explorer.  -->
     <script>
       var BLOCKLY_BOOTSTRAP_OPTIONS = {
-        additionalScripts: [
+        scripts: [
           'build/msg/en.js',
           'tests/playgrounds/screenshot.js',
           'node_modules/@blockly/dev-tools/dist/index.js',