Break paragraphs on blank lines, like Emacs

Previously multiple notional paragraphs could be rolled up into one
OrgParagraph. This is usually not a problem, but Emacs does bidi text direction
detection per-paragraph, so to make this possible we must have only one true
paragraph per OrgParagraph.

lib/src/org/grammar.dart

@@ -108,12 +108,15 @@ class OrgContentGrammarDefinition extends GrammarDefinition {
 
   Parser paragraph() =>
       ref0(indent).flatten('Paragraph indent expected') &
-      ref1(textRun, ref0(nonParagraphElements)).plusLazy(ref0(paragraphEnd));
+      ref1(textRun, ref0(nonParagraphElements) | newline().repeat(2))
+          .plusLazy(ref0(paragraphEnd)) &
+      ref0(blankLines).optional();
 
   Parser nonParagraphElements() =>
       element()..replace(ref0(paragraph), noOpFail());
 
-  Parser paragraphEnd() => endOfInput() | ref0(nonParagraphElements);
+  Parser paragraphEnd() =>
+      endOfInput() | newline().repeat(2) | ref0(nonParagraphElements);
 
   Parser textRun([Parser? limit]) => ref0(object) | ref1(plainText, limit);
 

lib/src/org/model.dart

@@ -1426,10 +1426,11 @@ class OrgListOrderedItem extends OrgListItem {
 }
 
 class OrgParagraph extends OrgParentNode {
-  OrgParagraph(this.indent, this.body, [super.id]);
+  OrgParagraph(this.indent, this.body, this.trailing, [super.id]);
 
   final String indent;
   final OrgContent body;
+  final String trailing;
 
   @override
   List<OrgNode> get children => [body];
@@ -1440,7 +1441,9 @@ class OrgParagraph extends OrgParentNode {
 
   @override
   bool contains(Pattern pattern) =>
-      indent.contains(pattern) || body.contains(pattern);
+      indent.contains(pattern) ||
+      body.contains(pattern) ||
+      trailing.contains(pattern);
 
   @override
   String toString() => 'OrgParagraph';
@@ -1449,17 +1452,20 @@ class OrgParagraph extends OrgParentNode {
   void _toMarkupImpl(OrgSerializer buf) {
     buf
       ..write(indent)
-      ..visit(body);
+      ..visit(body)
+      ..write(trailing);
   }
 
   OrgParagraph copyWith({
     String? indent,
     OrgContent? body,
+    String? trailing,
     String? id,
   }) =>
       OrgParagraph(
         indent ?? this.indent,
         body ?? this.body,
+        trailing ?? this.trailing,
         id ?? this.id,
       );
 }

lib/src/org/parser.dart

@@ -124,8 +124,9 @@ class OrgContentParserDefinition extends OrgContentGrammarDefinition {
   Parser paragraph() => super.paragraph().map((items) {
         final indent = items[0] as String;
         final bodyElements = items[1] as List;
+        final trailing = items[2] as String;
         final body = OrgContent(bodyElements.cast());
-        return OrgParagraph(indent, body);
+        return OrgParagraph(indent, body, trailing);
       });
 
   @override