Build: Create webpack stats target directory if needed and accept boolean flag

[ghengeveld]

Issue: -

What I did

Instead of writeFile this uses outputFile so that it will create directories if needed.
With this change, --webpack-stats-json won't require a value anymore. If the flag is present with no value (true), it will use the outputDir as its value.
Also I updated the args definitions to correctly use <> (required) and [] (optional)

How to test

• Is this testable with Jest or Chromatic screenshots? no
• Does this need a new example in the kitchen sink apps? no
• Does this need an update to the documentation? no

If your answer is yes to any of these, please make sure to include it in your PR.

[shilman]

LGTM 💯