[6.x] Allow PHP Tags in Antlers to Override any Variable

[JohnathonKoster]

This PR fixes #11222 by allowing developers to override any variable when using the {{? ... ?}} or {{$ $}} PHP tags. With the changes in this PR, the following will now be possible without having to explicitly assign the variable first:

{{?
    // This will now update $theVariableName without an explicit assignment.
    array_unshift($theVariableName, 'The Value');
?}}

{{ theVariableName}} ... {{ /theVariableName }}

While this is technically a bug fix, I am targeting 6.x+ as it is a significant behavior change that could impact existing projects.