[DowngradePhp80] Add DowngradeSubstrFalsyRector

[samsonasik]

Feature Request

Diff

-return substr('a', 2);
+return (string) substr('a', 2);

Add cast, to ensure it always string. Ref https://3v4l.org/PGpEe

This needs to identify its usage to avoid too many changes in many area.

[samsonasik]

I created PR

• [DowngradePhp80] Add DowngradeSubstrFalsyRector rector-downgrade-php#339

for it.