better parser errors

error_test/parser/function_errors.sui

@@ -0,0 +1,24 @@
+# Function syntax errors
+
+# Missing function name
+fn -> int
+    42
+
+# Missing parameter type annotation
+fn add(x, y: int) -> int
+    x + y
+
+# Missing comma in parameters
+fn add(x: int y: int) -> int
+    x + y
+
+# Missing closing parenthesis
+fn add(x: int, y: int -> int
+    x + y
+
+# Missing return type arrow
+fn add(x: int, y: int) int
+    x + y
+
+# Invalid function body (missing expression)
+fn test() -> int