update-api-docs.js: execa returns error if file path has space(s) in it

[nfmohit]

Describe the bug
This issue refers to the bin/api-docs/update-api-docs.js file. In this script, we're using execa to execute docgen, specifically in this line.

If your path to the WordPress installation has spaces in it, execa fails with an error code 127. When you run node ./bin/api-docs/update-api-docs.js, here's what the entire error message looks like:

Error: Command failed with exit code 127: /Users/nahid/Sites/Local by Flywheel/hdev-single/app/public/wp-content/plugins/gutenberg/node_modules/.bin/docgen packages/annotations/src/store/selectors.js --output docs/designers-developers/developers/data/data-core-annotations.md --to-token --use-token "Autogenerated selectors|../../../../packages/annotations/src/store/selectors.js" --ignore "/unstable|experimental/i"
/bin/sh: /Users/nahid/Sites/Local: No such file or directory
    at makeError (/Users/nahid/Sites/Local by Flywheel/hdev-single/app/public/wp-content/plugins/gutenberg/node_modules/execa/lib/error.js:58:11)
    at handlePromise (/Users/nahid/Sites/Local by Flywheel/hdev-single/app/public/wp-content/plugins/gutenberg/node_modules/execa/index.js:114:26)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async Transform.<anonymous> (/Users/nahid/Sites/Local by Flywheel/hdev-single/app/public/wp-content/plugins/gutenberg/bin/api-docs/update-api-docs.js:207:5) {
  shortMessage: 'Command failed with exit code 127: /Users/nahid/Sites/Local by Flywheel/hdev-single/app/public/wp-content/plugins/gutenberg/node_modules/.bin/docgen packages/annotations/src/store/selectors.js --output docs/designers-developers/developers/data/data-core-annotations.md --to-token --use-token "Autogenerated selectors|../../../../packages/annotations/src/store/selectors.js" --ignore "/unstable|experimental/i"',
  command: '/Users/nahid/Sites/Local by Flywheel/hdev-single/app/public/wp-content/plugins/gutenberg/node_modules/.bin/docgen packages/annotations/src/store/selectors.js --output docs/designers-developers/developers/data/data-core-annotations.md --to-token --use-token "Autogenerated selectors|../../../../packages/annotations/src/store/selectors.js" --ignore "/unstable|experimental/i"',
  exitCode: 127,
  signal: undefined,
  signalDescription: undefined,
  stdout: '',
  stderr: '/bin/sh: /Users/nahid/Sites/Local: No such file or directory',
  failed: true,
  timedOut: false,
  isCanceled: false,
  killed: false
}

As you can see in the above error message, this specific test site of mine exists in a folder which contains "Local by Flywheel", i.e. with spaces in it. I understand having such an environment/site path is not the best practice, but I think we should do something for contributors who does have such a setup.

I did some research and found a (sort of) relevant issue in execa: sindresorhus/execa#408. Then, I landed upon this issue in node: nodejs/node#6803 and upon further reading, it appears that it is a limitation within child_processes, and the documentation suggests quoting the path if it has spaces in it.

I'll submit a PR with the path quoted so that it works with such a file structure setup.

To reproduce
Steps to reproduce the behaviour:

1. Set up a local WordPress installation with spaces in its file path, for example, /Users/<username>/Sites/Word Press (ugh naughty!)
2. Set up the Gutenberg repository as a plugin in that installation.
3. Run node ./bin/api-docs/update-api-docs.js.
4. Reproduce the above-mentioned error.

Expected behavior
It shouldn't return such an error merely for the file path.

Editor version (please complete the following information):

• WordPress version: 5.4.1
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? "gutenberg plugin"
• If the Gutenberg plugin is installed, which version is it? 8.1.0

Desktop (please complete the following information):

• OS: macOS
• Browser: Google Chrome
• Version 81.0.4044.138

[nfmohit]

I've submitted #22513 aiming to address this.