syntax: support `elif` clauses for alternative `if` syntax

In the alternative `if` syntax mode, support a specific `elif` keyword instead
of requiring an `else` branch followed by a disjunct `if` statement.

The advantage is that templates do not require error-prone redundant `endif`
keywords in else-if ladders.

After this change, the following example:

    {% if (...): %}
    One condition
    {% else if (...): %}
    Another condition
    {% else if (...): %}
    A third condition
    {% else %}
    Final condition
    {% endif; endif; endif %}

... can be simplified into:

    {% if (...): %}
    One condition
    {% elif (...): %}
    Another condition
    {% elif (...): %}
    A third condition
    {% else %}
    Final condition
    {% endif %}

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

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