Add PHPStan support

[swissspidy]

It would be useful to add PHPStan static analysis support to this package and improve the code quality throughout the code base. This way the individual commands don't all have to set it up themselves and cause repetition.

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After landing the initial implementation and working on improving some of the main packages, here are a couple of observations / enhancement suggestions for stronger typing:

• Add dynamic return type extension for \WP_CLI\Utils\parse_url()
  Such an extension already exists for \parse_url() in PHPStan itself and \wp_parse_url() in the szepeviktor/phpstan-wordpress package (which is licensed under MIT, like WP-CLI).
• Add extension to type $args and $assoc_args based on the command's docblock.
  I think this would be really powerful and would make docblock maintenance easier. Not sure it's possible though, see Changing method parameter types based on PHPDoc contents phpstan/phpstan#13058
• Add dynamic return type extension for \WP_CLI\Utils\get_flag_value(), which currently returns mixed.
• Consider adopting phpstan/phpstan-strict-rules
• Consider adopting phpstan/phpstan-deprecation-rules
• Consider adopting swissspidy/phpstan-no-private
• Consider adopting phpstan/phpstan-phpunit
• Improve types for \WP_CLI\Utils\iterator_map
  Should use generics
• Add dynamic return type extension for \WP_CLI::runcommand()

[danielbachhuber]

Seems reasonable 👍

[swissspidy]

Reopening to discuss potential enhancements