New EnsureExplicitNullableTypes rule

[OskarStark]

Based on

• https://github.com/symfony/symfony-docs/pull/19786

1. It should be a LineContentRule
2. It should only run, when inside a PHP Code-Block
3. It should check all occurences via a regex if a property is = null, but missing a ?

You can take inspiration from

• https://github.com/OskarStark/doctor-rst/pull/1394