[eslint] Adds rule one-var and autofixes all related errors

[scinos]

Changes proposed in this Pull Request

• Enables estline rule one-var. It forbids patterns like:

  let var1, var2;
  

  and autofix them to

  let var1;
  let var2;
  

• Apply one-var autofix across the codebase.

• Run prettier in affected files.

• Manually fix https://github.com/Automattic/wp-calypso/pull/47310/files#diff-3ec97767860a9c864e12180449bc1777637d3861ece2fd2139fd12f335516c01 because it hit the bug [one-var] Autofixing this rule drops some exported vars eslint/eslint#13834

After applying this rule it should be easier to autofix no-var and prefer-const

Testing instructions

• Ignore code style errors, there will be a ton.
• This change should be safe, nothing to test. If the test are green it means that the app can be compiled and therefore the autofix didn't include any syntax error.

[matticbot]

Test live: https://calypso.live/?branch=add/eslint-one-var

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[tyxla]

Absolutely beautiful, thank you!

Let's 🚢 once checks are green before it gets stale again.

[matticbot]

Caution: This PR affects files in the Editing Toolkit Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D52603-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing the Editing Toolkit Plugin for more info: PCYsg-ly5-p2