Add parser support for the math helper.

src/parser/parser.rs

@@ -47,6 +47,7 @@ pub enum DustTag<'a> {
     DTHelperSelect(ParameterizedBlock<'a>),
     DTHelperAny(ParameterizedBlock<'a>),
     DTHelperNone(ParameterizedBlock<'a>),
+    DTHelperMath(ParameterizedBlock<'a>),
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -279,6 +280,10 @@ fn dust_tag_helper(i: &str) -> IResult<&str, DustTag> {
             parameterized_block("{@", &tag_to_path("none")),
             DustTag::DTHelperNone,
         ),
+        map(
+            parameterized_block("{@", &tag_to_path("math")),
+            DustTag::DTHelperMath,
+        ),
     ))(i)
 }