Isolate `$foreachVars` for `{include …}` calls

[dtdesign]

Nesting the same template inside a foreach loop that is also accessed inside the nested call will overwrite the values from the outer template due to identical identifiers being used.

This was partially addressed for the sandbox mode in #4445, but this does not take care of non-sandbox calls that are still affected by the underlying issue.

See https://github.com/WoltLab/WCF/pull/4445#issuecomment-895052103 for an idea for the isolation of $foreachVars.