Org reader: treat "abstract" block as metadata

A block of type "abstract" is assumed to define the document's abstract.
It is transferred from the main text to the metadata.

Closes: #8204

src/Text/Pandoc/Readers/Org/Blocks.hs

@@ -191,6 +191,7 @@ orgBlock = try $ do
       "quote"   -> parseBlockLines (fmap B.blockQuote)
       "verse"   -> verseBlock
       "src"     -> codeBlock blockAttrs
+      "abstract"-> metadataBlock
       _         -> parseBlockLines $
                    let (ident, classes, kv) = attrFromBlockAttributes blockAttrs
                    in fmap $ B.divWith (ident, classes ++ [blkType], kv)
@@ -290,6 +291,16 @@ verseBlock blockType = try $ do
      line <- parseFromString inlines (indentedLine <> "\n")
      return (trimInlinesF $ pure nbspIndent <> line)
 
+-- | Parses an environment of the given name and adds the result to the document
+-- metadata under a key of the same name.
+metadataBlock :: PandocMonad m => Text -> OrgParser m (F Blocks)
+metadataBlock blockType = try $ do
+  content <- parseBlockLines id blockType
+  meta'   <- orgStateMeta <$> getState
+  updateState $ \st ->
+    st { orgStateMeta = B.setMeta blockType <$> content <*> meta' }
+  return mempty
+
 -- | Read a code block and the associated results block if present.  Which of
 -- the blocks is included in the output is determined using the "exports"
 -- argument in the block header.

test/command/8204.md

@@ -1,3 +1,4 @@
+Include abstract in Org output.
 ```
 % pandoc --to=org --standalone
 ---
@@ -14,3 +15,25 @@ It has multiple paragraphs.
 
 #+end_abstract
 ```
+
+Parse abstract environment as *abstract* metadata field.
+```
+% pandoc --from=org --to=markdown --standalone
+#+begin_abstract
+
+This is an /abstract/.
+
+It has multiple paragraphs.
+#+end_abstract
+
+Main text
+^D
+---
+abstract: |
+  This is an *abstract*.
+
+  It has multiple paragraphs.
+---
+
+Main text
+```