CLI: Add additional files api to sb repro

[yannbf]

Issue: N/A

What I did

Added additionalFiles to repro config, so that we can add files before initializing Storybook.

Looks like this:

export const react = {
  framework: 'react',
  name: 'react',
  version: 'latest',
  generator: fromDeps('react', 'react-dom'),
  additionalDeps: ['prop-types'],
  additionalFiles: [
    { 
      path: '.babelrc',
      contents: dedent`
        {
          "presets": ["@babel/preset-env"]
        }
      `,
    }
  ]
};

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

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

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[codecov]

Codecov Report

Merging #18389 (4f5b90e) into next (95bd68d) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             next   #18389      +/-   ##
==========================================
- Coverage   32.05%   32.04%   -0.01%     
==========================================
  Files         975      975              
  Lines       19211    19216       +5     
  Branches     4031     4031              
==========================================
  Hits         6158     6158              
- Misses      12490    12494       +4     
- Partials      563      564       +1     

Impacted Files	Coverage Δ
lib/cli/src/repro-generators/configs.ts	0.00% <ø> (ø)
lib/cli/src/repro-generators/scripts.ts	0.00% <0.00%> (ø)
addons/a11y/src/components/A11YPanel.tsx	84.09% <0.00%> (ø)

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[shilman]

LGTM -- any idea what's up with the codecov comments in the review mode? it's really annoying

[yannbf]

LGTM -- any idea what's up with the codecov comments in the review mode? it's really annoying

Definitely annoying – I think the "problem" was introduced with the A11yPanel component changes from another PR: https://app.codecov.io/gh/storybookjs/storybook/compare/18389/changes

[shilman]

Looks great! 💯