message: refactor(docs-infra): replace `System.import()` with `import()`
  `System.import()` has been deprecated in webpack and produces the
  following warning when building the app:
  ```
  WARNING in ./src/app/custom-elements/code/pretty-printer.service.ts
  System.import() is deprecated and will be removed soon. Use import()
  instead.
  ```
  Switching to `import()` to get rid of the warning.
  Fixes
  Closes
  PR Close
diff: @@ -5,10 +5,6 @@ import { first, map, share } from 'rxjs/operators';
  import { Logger } from 'app/shared/logger.service';
  -declare const System: {
  - import(name: string): Promise<any>;
  -};
  -
  type PrettyPrintOne = (code: string, language?: string, linenums?: number | boolean) => string;
  /**
  @@ -26,8 +22,9 @@ export class PrettyPrinter {
  private getPrettyPrintOne(): Promise<PrettyPrintOne> {
  const ppo = (window as any)['prettyPrintOne'];
  return ppo ? Promise.resolve(ppo) :
  - // prettify.js is not in window global; load it with webpack loader
  - System.import('assets/js/prettify.js')
  + // `prettyPrintOne` is not on `window`, which means `prettify.js` has not been loaded yet.
  + // Import it; ad a side-effect it will add `prettyPrintOne` on `window`.
  + import('assets/js/prettify.js' as any)
  .then(
  () => (window as any)['prettyPrintOne'],
  err => {
  