Implement the new fields for export snippet.

src/parser/export_snippet.rs

@@ -39,7 +39,7 @@ pub(crate) fn export_snippet<'b, 'g, 'r, 's>(
         parser_with_context!(contents)(&parser_context),
     )))(remaining)?;
     let (remaining, _) = tag("@@")(remaining)?;
-    let (remaining, _trailing_whitespace) =
+    let (remaining, post_blank) =
         maybe_consume_object_trailing_whitespace_if_not_exiting(context, remaining)?;
     let source = get_consumed(input, remaining);
     Ok((
@@ -48,6 +48,7 @@ pub(crate) fn export_snippet<'b, 'g, 'r, 's>(
             source: source.into(),
             backend: backend_name.into(),
             contents: backend_contents.map(|(_colon, backend_contents)| backend_contents.into()),
+            post_blank: post_blank.map(Into::<&str>::into),
         },
     ))
 }

src/types/object.rs

@@ -206,6 +206,7 @@ pub struct ExportSnippet<'s> {
     pub source: &'s str,
     pub backend: &'s str,
     pub contents: Option<&'s str>,
+    pub post_blank: Option<&'s str>,
 }
 
 #[derive(Debug)]
@@ -802,11 +803,15 @@ impl<'s> StandardProperties<'s> for ExportSnippet<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        None
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        self.post_blank
+            .map(|text| text.chars().count())
+            .unwrap_or(0)
+            .try_into()
+            .expect("Too much post-blank to fit into a PostBlank.")
     }
 }