[feature/template-engine] Allow leading underscores in variable names.

Subsilver uses ._file in overall_header.

PHPBB3-9726

tests/template/template_test.php

@@ -235,6 +235,15 @@ class phpbb_template_template_test extends phpbb_test_case
 				array('loop.inner'),
 				"first\n0\n0\n2\nx\nset\n1\n1\n2\ny\nset\nlast",
 			),*/
+			array(
+				// Just like a regular loop but the name begins
+				// with an underscore
+				'loop_underscore.html',
+				array(),
+				array(),
+				array(),
+				"noloop\nnoloop",
+			),
 			array(
 				'lang.html',
 				array(),

tests/template/templates/loop_underscore.html

@@ -0,0 +1,21 @@
+<!-- BEGIN _underscore_loop -->
+loop
+<!-- BEGINELSE -->
+noloop
+<!-- END loop -->
+
+<!-- IF ._underscore_loop -->
+loop
+<!-- ELSE -->
+noloop
+<!-- ENDIF -->
+
+<!-- IF ._underscore_loop == 2 -->
+loop
+<!-- ENDIF -->
+
+<!-- BEGIN _underscore_loop -->
+<!-- BEGIN !block -->
+loop#{loop.S_ROW_COUNT}-block#{block.S_ROW_COUNT}
+<!-- END !block -->
+<!-- END _underscore_loop -->