Support boolean literals

This will transparently convert them into an actual boolean which
should skip any variable-like parsing that would otherwise be done.

tatsu_grammar.txt

@@ -76,7 +76,7 @@ variable_close
 
 variable_identifier
     =
-    ( IDENTIFIER | LITERAL )
+    ( LITERAL | IDENTIFIER )
     { variable_accessor }*
     ;
 
@@ -150,7 +150,7 @@ content                 = !(block_open | variable_open | comment_open) CHAR ;
 
 LITERAL
     =
-    STRING_LITERAL | NUMBER_LITERAL
+    STRING_LITERAL | NUMBER_LITERAL | BOOLEAN_LITERAL
     ;
 
 INTEGER_LITERAL
@@ -180,6 +180,11 @@ STRING_LITERAL_DOUBLE_QUOTE
     '"' { !'"' /./ }* '"'
     ;
 
+BOOLEAN_LITERAL
+    =
+    (("true" | "True") @:`True`) | (("false" | "False") @:`False`)
+    ;
+
 IDENTIFIER
     =
     /[a-zA-Z_][a-zA-Z0-9_]*/

test_template.jinja

@@ -11,5 +11,7 @@ across lines #}
 {% raw %}
     {% block %}fake content{{ fake vars }}
 {% endraw %}
+{% if False %}
 {{ '{{' }}
+{% endif %}
 {% trans user=user.username %}Hello, {{ user }}!{% endtrans %}