Add a test case showing only paragraphs exist on the first line for plain lists.

src/parser/plain_list.rs

@@ -1 +1,11 @@
+use super::error::Res;
+use super::lesser_element::Paragraph;
+use super::Context;
 
+#[allow(dead_code)]
+pub fn plain_list_item<'r, 's>(
+    context: Context<'r, 's>,
+    input: &'s str,
+) -> Res<&'s str, Paragraph<'s>> {
+    todo!()
+}