New: Add hot module reloading for theme and styles

[elevatebart]

closes #1475

This Pull Request allows users of react-styleguidist to iterate faster with the styling of their guides.
It avoids having to stop and restart the webpack-dev-server every time a change is made to the theme or the styles.

To achieve this goal, I allow the styles and theme parameters to be passed as File paths instead of an object. Webpack can then use the new files as modules and hot replace them.

• create a themed example
• adapt the schema/config.ts
• Add documentation about new feature
• change styleguide-loader.ts to resolve the style files using requireIt and set them up for hmr using adddependency and the hmrStyles flag
  • add tests
• update client/index.ts to resolve es6 & es5 exports of style files and pass them on to renderStyleguide
• add a key to Styleguide on account of the calculated cssRevision to force its refresh and recalc the styles
• use the new context to cache-bust createStyleSheet memoization when styles have
  changed
  • add tests

[elevatebart]

I thought that I could find a simple way to decouple components from the styles and themes object. I could not without big changes to the rsg-components files.

To have Webpack recompile each component's style independently (and therefore hot replace only the components impacted) the requests to the Styled component would have to be different per component.

This is too big of a change for what I want for this PR. I will come up with another one.

This one is ready for review.

[sapegin]

👾

[sapegin]

And there are a few unresolved comments from the previous iteration.

[elevatebart]

Finally done all the last fixes, we should be closer to the finish line

[sapegin]

There are still couple of open comments in styleguide-loader.ts, the rest looks good!

[elevatebart]

@sapegin I did it all now, finally, I am open to questions.

[elevatebart]

@sapegin is this comment better ?

[elevatebart]

@sapegin do you think I can merge this one?

[sapegin]

Yup, go for it!

[styleguidist-bot]

🎉 This PR is included in version 10.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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