Storyshots addon refactoring

[igor-dv]

Issue: including changed needed for #2517 (probably for #3152 too)

This PR is rebased on "storyshots-separation" branch because I didn't want to wait for the #3742 to be merged.

What I did

1. Created and exposed a class called Stories2SnapsConverter in order to map story-files to snapshot-files and vise versa.
2. Refactored a structure of the addon to make it more maintainable/readable.
3. Fixed Storyshots with Storybook 4 for React fails with Angular import. #3754

[codecov]

Codecov Report

Merging #3745 into master will increase coverage by 0.3%.
The diff coverage is 87.01%.

@@            Coverage Diff            @@
##           master    #3745     +/-   ##
=========================================
+ Coverage   41.23%   41.53%   +0.3%     
=========================================
  Files         459      461      +2     
  Lines        5100     5128     +28     
  Branches      860      860             
=========================================
+ Hits         2103     2130     +27     
- Misses       2482     2483      +1     
  Partials      515      515

Impacted Files	Coverage Δ
...s/storyshots-core/src/frameworks/vue/renderTree.js	0% <ø> (ø)
...ots-core/src/frameworks/react/renderShallowTree.js	100% <ø> (ø)
.../storyshots-core/src/frameworks/html/renderTree.js	0% <ø> (ø)
...storyshots-core/src/frameworks/react/renderTree.js	100% <ø> (ø)
...ots/storyshots-core/src/api/getIntegrityOptions.js	85.71% <ø> (ø)
...oryshots-core/src/frameworks/angular/renderTree.js	0% <ø> (ø)
.../storyshots-core/src/frameworks/require_context.js	91.66% <ø> (ø)
...yshots/storyshots-core/src/frameworks/rn/loader.js	0% <ø> (ø)
...ts/storyshots-core/src/frameworks/hasDependency.js	50% <ø> (ø)
...s/storyshots-core/src/frameworks/angular/loader.js	12.5% <0%> (ø)
... and 17 more

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[Hypnosphi]

LMK when it's ready for review

[igor-dv]

I think it's ready. Will work on other stuff in smaller PRs.

[Hypnosphi]

It's probably worth mentioning in MIGRATION.md, as well as puppeteer moving to other package

[igor-dv]

@Hypnosphi, can I merge this?