markdown: support custom header color

crates/languages/src/markdown/highlights.scm

@@ -11,6 +11,32 @@
   (list_marker_parenthesis)
 ] @punctuation.list_marker
 
+(setext_heading
+  (paragraph) @markup.heading.1
+  (setext_h1_underline) @markup.heading.1)
+
+(setext_heading
+  (paragraph) @markup.heading.2
+  (setext_h2_underline) @markup.heading.2)
+
+(atx_heading
+  (atx_h1_marker)) @markup.heading.1
+
+(atx_heading
+  (atx_h2_marker)) @markup.heading.2
+
+(atx_heading
+  (atx_h3_marker)) @markup.heading.3
+
+(atx_heading
+  (atx_h4_marker)) @markup.heading.4
+
+(atx_heading
+  (atx_h5_marker)) @markup.heading.5
+
+(atx_heading
+  (atx_h6_marker)) @markup.heading.6
+
 (fenced_code_block
   (info_string
     (language) @text.literal))