fix: link fragment with accents should be valid if the heading exists

Fixes a regression introduced in v2.3.0, which needed to lower case/manage case insensitive heading.

src/utils.js

@@ -26,7 +26,7 @@ const convertHeadingToHTMLFragment = (inlineText) => {
           "",
         )
         .replace(/ /gu, "-"),
-    )
+    ).toLowerCase()
   )
 }