[fine] Implement the wildcard pattern

fine/src/tokens.rs

@@ -57,6 +57,7 @@ pub enum TokenKind {
     Select,
     Selff,
     True,
+    Underscore,
     While,
     Yield,
 }
@@ -351,6 +352,11 @@ impl<'a> Tokens<'a> {
                     return TokenKind::Yield;
                 }
             }
+            '_' => {
+                if ident == "_" {
+                    return TokenKind::Underscore;
+                }
+            }
             _ => (),
         }
 
@@ -605,10 +611,11 @@ mod tests {
 
     test_tokens!(
         more_more_keywords,
-        "in is match",
+        "in is match _",
         (0, In, "in"),
         (3, Is, "is"),
-        (6, Match, "match")
+        (6, Match, "match"),
+        (12, Underscore, "_")
     );
 
     test_tokens!(