fix: fix unexpected setext heading (#2986)

Author: UziTech

src/rules.ts

@@ -87,7 +87,7 @@ export const block: Record<BlockRuleNames, Rule> & Record<BlockSubRuleNames, Rul
     + ')',
   def: /^ {0,3}\[(label)\]: *(?:\n *)?([^<\s][^\s]*|<.*?>)(?:(?: +(?:\n *)?| *\n *)(title))? *(?:\n+|$)/,
   table: noopTest,
-  lheading: /^((?:(?!^bull ).|\n(?!\n|bull ))+?)\n {0,3}(=+|-+) *(?:\n+|$)/,
+  lheading: /^(?!bull )((?:.|\n(?!\s*?\n|bull ))+?)\n {0,3}(=+|-+) *(?:\n+|$)/,
   // regex template, placeholders will be replaced according to different paragraph
   // interruption rules of commonmark and the original markdown spec:
   _paragraph: /^([^\n]+(?:\n(?!hr|heading|lheading|blockquote|fences|list|html|table| +\n)[^\n]+)*)/,

test/specs/new/setext_blankline.html

@@ -0,0 +1,9 @@
+<p>a</p>
+<p>b</p>
+<p>=</p>
+
+<p>a</p>
+<p>b</p>
+<ul>
+  <li></li>
+</ul>

test/specs/new/setext_blankline.md

@@ -0,0 +1,11 @@
+a
+ 
+b
+ 
+=
+
+a
+ 
+b
+ 
+-