Create structure for plain links.

src/parser/plain_link.rs

@@ -0,0 +1,10 @@
+use super::Context;
+use crate::error::Res;
+use crate::parser::object::PlainLink;
+use crate::parser::util::not_yet_implemented;
+
+#[tracing::instrument(ret, level = "debug")]
+pub fn plain_link<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, PlainLink<'s>> {
+    not_yet_implemented()?;
+    todo!();
+}