Add failing tests (fmt panic) on weird doc comments.

Isolated doc comments causes the compiler to panic with:

  ```
  'no consecutive empty lines'
  ```

  This is reproducible when doc comments are wrapped in sandwich between
  comments and newlines.

crates/aiken-lang/src/tests/check.rs

@@ -322,6 +322,25 @@ fn utf8_hex_literal_warning() {
     ))
 }
 
+#[test]
+fn weird_comments() {
+    let source_code = r#"
+        // A
+
+        /// B
+
+        // C
+        fn foo () {
+          todo
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::UnexpectedDocComment { .. }))
+    ))
+}
+
 #[test]
 fn discarded_let_bindings() {
     let source_code = r#"

crates/aiken-lang/src/tests/format.rs

@@ -629,3 +629,29 @@ fn test_preserve_pipe() {
 
     assert_fmt(src, expected);
 }
+
+#[test]
+fn weird_comments() {
+    let src = indoc! { r#"
+        // A
+
+        /// B
+
+        /// C
+        fn bar () {
+          todo
+        }
+    "#};
+
+    let expected = indoc! { r#"
+        // A
+
+        /// B
+        /// C
+        fn bar () {
+          todo
+        }
+    "#};
+
+    assert_fmt(src, expected);
+}

crates/aiken-lang/src/tipo/error.rs

@@ -892,6 +892,13 @@ The best thing to do from here is to remove it."#))]
         #[label("{} arguments", if *count < 2 { "not enough" } else { "too many" })]
         location: Span,
     },
+
+    #[error("I ran into a lonely doc comment not attached to any function definition.")]
+    #[diagnostic(code("unexpected::doc_comment"))]
+    UnexpectedDocComment {
+        #[label("unexpected doc comment")]
+        location: Span,
+    },
 }
 
 impl Error {