[DowngradePhp80] Add `DowngradeStringableInterfaceRector`

[osbre]

PHP 8 added the \Stringable interface.

Downgrade rule implementation would include:

• Removal of implements \Stringable from classes.
• Replacement of the expression $a instanceof \Stringable to something like method_exists($a, '__toString')

Refs:

• https://www.php.net/manual/en/class.stringable.php
• https://grep.app/search?q=instanceof%20Stringable
• https://grep.app/search?q=implements%20Stringable

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Perhaps a "feature" label can be added to this issue.

[samsonasik]

For remove implements Stringable, it actually already applied by configurable RemoveInterfacesRector

https://github.com/rectorphp/rector-downgrade-php/blob/4228a7305efbd92687cf7f6cbf7b683a28412eb1/config/set/downgrade-php80.php#L44-L48

Ref https://getrector.com/demo/47d27225-1e6b-4855-91d5-9d6c4a1c4a14

The instanceof Stringable check is that what we need, as currently still not applied

Ref https://getrector.com/demo/d29a298b-8bbc-467c-ae35-9e2c91163b9d