Support {% if ...; end; ... %} macro expressions

[HertzDevil]

Follow-up to #14505. The following is already valid: (this is not a MacroIf node, but a MacroExpression node whose first sub-expression is an If, because the if is closed within the braces)

{%
  2 if 1
  3
%}

This PR adds support for the equivalent with a non-trailing if:

{%
  if 1
    2
  end
  3
%}

In fact, macro interpolation already transforms the former into the latter; these previously broken interpolations will now work.

{% unless ...; end; ... %} is also handled in the same way.

{{ ... }} expects a single expression and never an expression list in the first place, so that form is unaffected.