support .vue extention resolving

[kazupon]

Issue:
In Vue projects, Cannot import single file components (.vue) without extension.

What I did

add .vue extention to resolve.extensions of webpack config.

How to test

Is this testable with jest or storyshots?
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.

[codecov]

Codecov Report

Merging #2896 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2896   +/-   ##
=======================================
  Coverage   37.37%   37.37%           
=======================================
  Files         426      426           
  Lines        9151     9151           
  Branches      871      871           
=======================================
  Hits         3420     3420           
- Misses       5196     5197    +1     
+ Partials      535      534    -1

Impacted Files	Coverage Δ
app/vue/src/server/config/webpack.config.prod.js	0% <ø> (ø)	⬆️
app/vue/src/server/config/webpack.config.js	0% <ø> (ø)	⬆️
app/polymer/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/polymer/src/server/babel_config.js	0% <0%> (-76.67%)	⬇️
addons/storyshots/src/react/renderShallowTree.js	57.14% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/shortcuts_help.js	25% <0%> (ø)	⬆️
.../ui/components/stories_panel/stories_tree/index.js	38.73% <0%> (ø)	⬆️
lib/core/src/client/preview/story_store.js	84.21% <0%> (ø)	⬆️
...ddons/actions/src/containers/ActionLogger/index.js	49.25% <0%> (ø)	⬆️
...i/src/modules/ui/components/stories_panel/index.js	20.96% <0%> (ø)	⬆️
... and 51 more

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