Adopt and enforce new `WP_CLI_CS` standard

[schlessera]

We have a new PHPCS standard for WP-CLI called WPCliCS (props @jrfnl). It is part of the wp-cli/wp-cli-tests package starting with version v2.1.0.

To adopt & enforce this new standard, the following actions need to be taken for this repository:

• [x] Create a PR that adds a custom ruleset phpcs.xml.dist to the repository

  • Add phpcs.xml.dist file
  • Adapt .distignore to ignore phpcs.xml.dist & phpunit.xml.dist
  • Adapt .gitignore to ignore phpunit.xml, phpcs.xml & .phpcs.xml
  • Require version ^2.1 of the wp-cli/wp-cli-tests as a dev dependency
• [x] Make any required changes to the code that fail the checks from the above ruleset in separate PRs
• [x] Merge thre ruleset once all required changes have been processed and merged

A sample PR for a simple repository can be seen here: https://github.com/wp-cli/maintenance-mode-command/pull/3/files

Related https://github.com/wp-cli/wp-cli/issues/5179