Markdown reader: fenced code block shortcuts with attributes (#8174)

This allows the combination of the fenced code block shortcut form with
attributes:

````
```haskell {.class #id}
```
````

The code syntax class will be combined with the attribute classes.

This syntax allows for more intuitive writing and for better compatibility
with other Markdown parsers such as GitHub or Codeberg.

Closes #8174.

MANUAL.txt

@@ -3857,6 +3857,18 @@ This is equivalent to:
     qsort [] = []
     ```
 
+This shortcut form may be combined with attributes:
+
+    ```haskell {.numberLines}
+    qsort [] = []
+    ```
+
+Which is equivalent to:
+
+    ``` {.haskell .numberLines}
+    qsort [] = []
+    ```
+
 If the `fenced_code_attributes` extension is disabled, but
 input contains class attribute(s) for the code block, the first
 class attribute will be printed after the opening fence as a bare

src/Text/Pandoc/Readers/Markdown.hs

@@ -688,9 +688,13 @@ codeBlockFenced = try $ do
   rawattr <-
      (Left <$> (guardEnabled Ext_raw_attribute >> try rawAttribute))
     <|>
-     (Right <$> option ("",[],[])
-         ((guardEnabled Ext_fenced_code_attributes >> try attributes)
-          <|> ((\x -> ("",[toLanguageId x],[])) <$> many1Char nonspaceChar)))
+     (Right <$> (do
+           languageId <- option Nothing (Just . toLanguageId <$> try (many1Char $ satisfy (\x -> x `notElem` ['`', '{', '}'] && not (isSpace x))))
+           skipMany spaceChar
+           maybeAttr <- option Nothing (Just <$> (guardEnabled Ext_fenced_code_attributes >> try attributes))
+           return $ case maybeAttr of
+              Nothing -> ("", maybeToList languageId, [])
+              Just (elementId, classes, attrs) -> (elementId, maybe classes (: classes) languageId, attrs)))
   blankline
   contents <- T.intercalate "\n" <$>
                  manyTill (gobbleAtMostSpaces indentLevel >> anyLine)

test/command/8174.md

@@ -0,0 +1,44 @@
+````
+% pandoc -t native
+```yaml {#id}
+some: code
+```
+^D
+[ CodeBlock ( "id" , [ "yaml" ] , [] ) "some: code" ]
+````
+
+````
+% pandoc -t native
+```yaml {.class #id}
+some: code
+```
+^D
+[ CodeBlock
+    ( "id" , [ "yaml" , "class" ] , [] ) "some: code"
+]
+````
+
+<!-- Inline code sections at the start of the line should not be mistaken for code blocks -->
+````
+% pandoc -t native
+```ab```
+^D
+[ Para [ Code ( "" , [] , [] ) "ab" ] ]
+````
+
+````
+% pandoc -t native
+```ab```{.class}
+^D
+[ Para [ Code ( "" , [ "class" ] , [] ) "ab" ] ]
+````
+
+<!-- Illegal language identifiers should be treated as inline code for now -->
+````
+% pandoc -t native
+``` foo}{.bar}
+test
+```
+^D
+[ Para [ Code ( "" , [] , [] ) "foo}{.bar} test" ] ]
+````