Implement Rector rules to downgrade PHP code across versions

[leoloso]

Feature Request

Rector currently has 16 rules to downgrade PHP code across versions:

• DowngradePhp80
• DowngradePhp74
• DowngradePhp72
• DowngradePhp71

Through these rules, the plugin can be coded using features from PHP 8.0, and be transpiled to run on PHP 7.1. Currently, the GraphQL API for WP uses PHP 7.4 (the list of supported features is this one), and will switch to PHP 8.0 as soon as it's released.

There are an additional 22 downgrade rules, already documented but not yet implemented.

Implement them (in the Rector repo).

Context

The new rules will unlock pretty much all features from PHP 7.1/7.2/7.3/7.4/8.0, so they can be used in the plugin (eg: match expressions).