Add import to webpack extends example

[jjwon0]

Issue:

What I did

Update code sample in docs

How to test

Build website

Is this testable with jest or storyshots?

Don't think so?

Does this need a new example in the kitchen sink apps?

Don't think so?

Does this need an update to the documentation?

N/A

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

[codecov]

Codecov Report

Merging #2330 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2330   +/-   ##
=======================================
  Coverage   21.19%   21.19%           
=======================================
  Files         283      283           
  Lines        6156     6156           
  Branches      724      729    +5     
=======================================
  Hits         1305     1305           
+ Misses       4310     4296   -14     
- Partials      541      555   +14

Impacted Files	Coverage Δ
app/react/src/server/utils.js	0% <0%> (-53.58%)	⬇️
lib/ui/src/modules/ui/configs/handle_routing.js	24.73% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/init_panels.js	25% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Item.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
addons/knobs/src/components/types/Object.js	5.88% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/layout.js	12.5% <0%> (ø)	⬆️
...s/left_panel/stories_tree/tree_decorators_utils.js	45.23% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	30% <0%> (ø)	⬆️
... and 16 more

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[Hypnosphi]

Thanks @jjwon0!