Fix option lists with additional indentation in Markdown (#21633)

This is more Markdown-ish way to fix issue #21055, then PR #21625.
It does not enable RST definition lists, instead it makes
adding additional indentation (less than 4) right after a paragraph
be ignored, as it's done for all block elements in Markdown.
(In this case this tenet is applied to option lists that are not
part of CommonMark spec by themselves).

lib/packages/docutils/rst.nim

@@ -2447,7 +2447,9 @@ proc parseParagraph(p: var RstParser, result: PRstNode) =
           result.addIfNotNil(parseLineBlock(p))
         of rnMarkdownBlockQuote:
           result.addIfNotNil(parseMarkdownBlockQuote(p))
-        else: break
+        else:
+          dec p.idx  # allow subsequent block to be parsed as another section
+          break
       else:
         break
     of tkPunct:

tests/stdlib/trst.nim

@@ -689,6 +689,32 @@ suite "RST parsing":
                 rnLeaf  'desc2'
       """)
 
+  test "definition lists work correctly with additional indentation in Markdown":
+    check(dedent"""
+        Paragraph:
+          -c  desc1
+          -b  desc2
+        """.toAst() ==
+      dedent"""
+        rnInner
+          rnInner
+            rnLeaf  'Paragraph'
+            rnLeaf  ':'
+          rnOptionList
+            rnOptionListItem  order=1
+              rnOptionGroup
+                rnLeaf  '-'
+                rnLeaf  'c'
+              rnDescription
+                rnLeaf  'desc1'
+            rnOptionListItem  order=2
+              rnOptionGroup
+                rnLeaf  '-'
+                rnLeaf  'b'
+              rnDescription
+                rnLeaf  'desc2'
+      """)
+
   test "RST comment":
     check(dedent"""
         .. comment1