Feat: Add support for Flow to @addons/storysource

[Nemvts]

Issue: Feat: #4955

What I did

Add Prettier Flow parser to @addons/storysource

flow is now an available option for parser under the loader options

How to test

I added some flow type annotations to a few kitchen sink app stories to ensure that storybook was able to launch and storysource was able to parse Flow's syntax.

• Is this testable with Jest or Chromatic screenshots?
  Can create a mock file to test the inject decorator with flow stories

• Does this need a new example in the kitchen sink apps?
  No

• Does this need an update to the documentation?
  Yes, include flow as an available option.

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

[codecov]

Codecov Report

Merging #4971 into next will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             next    #4971      +/-   ##
==========================================
+ Coverage   35.06%   35.11%   +0.04%     
==========================================
  Files         563      564       +1     
  Lines        6975     6978       +3     
  Branches      934      935       +1     
==========================================
+ Hits         2446     2450       +4     
+ Misses       4034     4033       -1     
  Partials      495      495

Impacted Files	Coverage Δ
addons/storysource/src/loader/parsers/index.js	85.71% <100%> (+5.71%)	⬆️
...dons/storysource/src/loader/parsers/parser-flow.js	100% <100%> (ø)
addons/storysource/src/loader/parse-helpers.js	88.09% <0%> (+2.38%)	⬆️

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[igor-dv]

LGTM, but can you please add some flow usecase to https://github.com/storybooks/storybook/blob/next/addons/storysource/src/loader/inject-decorator.test.js?

[Nemvts]

I have added unit tests for the flow usecase and the code coverage looks good now.

[igor-dv]

Can you please merge from the next branch. Should solve the CI problems

[ndelangen]

I merged in next for you

[ndelangen]

@igor-dv Do you feel strongly about #4971 (review) ?

We could merge this and add the use-case later? I'll leave it up to you to merge.

[igor-dv]

The tests were added so 👌