Start the plain list module.

src/parser/plain_list.rs

@@ -0,0 +1,8 @@
+use super::error::Res;
+use super::token::PlainList;
+use super::Context;
+
+pub fn plain_list<'r, 's>(context: Context<'r, 's>, i: &'s str) -> Res<&'s str, PlainList<'s>> {
+    // todo
+    todo!()
+}