ci: implement phpcs, following the wp-graphql implementation

[jasonbahl]

• adds composer dev dependencies and scripts for phpcs and phpcbf
• adds phpcs.xml.dist from wp-graphql with changes per existing files in this plugin
• updates plugin header
• updates formatting to be phpcs compliant
• adds tests/_output directory for caching output of test and scripts like phpcs
• updates composer.lock file
• adds github workflow to check the code standards when PRs are opened

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closes #56