feat: use require to support config via cypress.js

[lmiller1990]

User facing changelog

• You may now write your config as cypress.js instead of cypress.json. cypress.js must confirm to the same API, for example:

module.exports = {
  video: false
}

Additional details

The eventual goal is outlined in this ticket. We need to support runner specific config files; JS is more flexible than JSON for this. The first step to supporting JS as well as JSON for configuration is to load it via require instead of fs.readJson.

How has the user experience changed?

It is still the same.

PR Tasks

N/A

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[cypress]

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Test summary

182 • 0 • 5 • 0 • 1

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Run details

Project	cypress
Status	Passed
Commit	efad917
Started	Mar 1, 2021 7:15 AM
Ended	Mar 1, 2021 7:23 AM
Duration	07:56 💡
OS	Linux Debian - 10.5
Browser	Chrome 83

View run in Cypress Dashboard ➡️

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Flakiness

	cypress/integration/retries.ui.spec.js		1
1	runner/cypress retries.ui.spec > opens attempt on each attempt failure for the screenshot, and closes after test passes

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[chrisbreiding]

We had to revert this for 6.7.1 in #15499 because it caused this issue: #15497.

The reason is that with the use of require, Node caches the contents of the cypress.json and we mutate the object, so subsequent requires don't read the actual contents of the file, but read from the mutated cache. I've fixed the mutation issue, since that's separate and should be fixed. But we still shouldn't use require for reading cypress.json because of the caching. If the user changes their cypress.json contents in their editor, we won't be able to read them with require because Node will always return the cached contents and not read from disk.