feat(npm/react): allow custom webpack config path for react-scripts plugin

[pashidlos]

• Closes Component tests. plugins/react-scripts. Allow configure of webpack config path #16480

User facing changelog

react-scripts plugin now could be configured with custom webpack config path

const injectDevServer = require('@cypress/react/plugins/react-scripts')
const path = require('path')

module.exports = (on, config) => {
  injectDevServer(
    on, 
    config, { 
      webpackConfigPath: './your/custom/path/webpack.config.js'  // <- added this option
    })

  return config
}

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[lmiller1990]

Code seems generally fine. Two comments:

• have you tested this? I see we have not actually added a test. I wonder if we could add a simple unit test, perhaps? I can probably look into this if you aren't sure how or don't have the time. You'd just write it with mocha - two use cases would be loading w/ default location, and loading w/ a custom location.
• if you want this to go out asap, it'll need to target master. Otherwise it will need to wait for the next release (every 2 weeks).

Please let me know

1. do you plan to retarget for master, or happy to wait for 2 weeks?
2. about adding a unit test - can you do this, or do you need me to look into this?
3. have you actually tested this in some capacity, or should I do a manually QA on top of the unit test we should add before merging?

Thanks!

[pashidlos]

@lmiller1990 thanks for quick response

• do you plan to retarget for master, or happy to wait for 2 weeks?

retargeted to master, unfortunately passing pipeline is a challenge due to unstable tests

• about adding a unit test - can you do this, or do you need me to look into this?

I wasn't able to find any example of those (should I add new npm script?)
please, assist with unit tests

• have you actually tested this in some capacity, or should I do a manually QA on top of the unit test we should add before merging?

I checked it manually and the default behaviour is covered with examples projects AFAIK