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>

1 files changed, 1 insertions, 0 deletions

diff --git a/lexer.c b/lexer.c
index eb513b8..f14a944 100644
--- a/lexer.c
+++ b/lexer.c
@@ -142,6 +142,7 @@ static const struct token reserved_words[] = {
 	{ T_CATCH,		"catch", 5 },
 	{ T_BOOL,		"false", 5, { .b = false } },
 	{ T_BOOL,		"true",  4, { .b = true } },
+	{ T_ELIF,		"elif",  4 },
 	{ T_ELSE,		"else",  4 },
 	{ T_THIS,		"this",  4 },
 	{ T_NULL,		"null",  4 },