feat: stricter validation of heading fragments by being Case sensitive

Fixes #8

BREAKING CHANGE: Heading fragments is now Case sensitive.
For example "#ExistIng-Heading" is invalid as it should be "#existing-heading".

src/utils.js

@@ -28,7 +28,7 @@ const convertHeadingToHTMLFragment = (inlineText) => {
           "",
         )
         .replace(/ /gu, "-"),
-    ).toLowerCase()
+    )
   )
 }