Allow 'panic' as a function application

This matches the same syntax as 'todo'.

Closes #82

grammar.js

@@ -357,7 +357,15 @@ module.exports = grammar({
         )
       ),
     panic: ($) =>
-      seq("panic", optional(seq("as", field("message", $._expression)))),
+      seq(
+        "panic",
+        optional(
+          choice(
+            seq("(", field("message", $.string), ")"),
+            seq("as", field("message", $._expression))
+          )
+        )
+      ),
     tuple: ($) => seq("#", "(", optional(series_of($._expression, ",")), ")"),
     list: ($) =>
       seq(

test/corpus/expressions.txt

@@ -146,6 +146,23 @@ panic as { "Hello, " <> "world!" }
         (string
           (quoted_content))))))
 
+================================================================================
+Todo and panic in function application style
+================================================================================
+
+todo("don't panic")
+panic("aaaah!")
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (todo
+    (string
+      (quoted_content)))
+  (panic
+    (string
+      (quoted_content))))
+
 ================================================================================
 Nested field access
 ================================================================================