Org template: add abstract from metadata as block of type "abstract"

See #8204.

data/templates/default.org

@@ -13,6 +13,11 @@ $for(header-includes)$
 $header-includes$
 
 $endfor$
+$if(abstract)$
+#+begin_abstract
+$abstract$
+#+end_abstract
+$endif$
 $for(include-before)$
 $include-before$
 

test/command/8204.md

@@ -0,0 +1,16 @@
+```
+% pandoc --to=org --standalone
+---
+abstract: |
+  This is an *abstract*.
+
+  It has multiple paragraphs.
+---
+^D
+#+begin_abstract
+This is an /abstract/.
+
+It has multiple paragraphs.
+
+#+end_abstract
+```