minor: SymfonySet - Add SimpleToComplexStringVariableFixer

[SpacePossum]

following the request here #6420

[coveralls]

Coverage remained the same at 92.873% when pulling 53fbefd on SpacePossum:6420 into 7936fb2 on FriendsOfPHP:master.

[derrabus]

It would probably make sense to create a new ruleset @PHP82Migration and include the fixer there as well.

[SpacePossum]

Good idea :) I'll compile a todo list/meta ticket for PHP8.2 👍

[SpacePossum]

Can this one maybe merged @julienfalque ?
It is not a big PR and its changes are approved by SF community representation and will also help a lot of people migrating towards PHP 8.2.

For me it would also be nice because there are more PR's that will get into some merge conflict cases as this is merged which I can than resolve.

Thanks for considering and your efforts into this :)

[julienfalque]

Thank you @SpacePossum.