Update mdast-util-toc

Author: wooorm

package.json

@@ -32,7 +32,7 @@
     "index.js"
   ],
   "dependencies": {
-    "mdast-util-toc": "^3.0.0"
+    "mdast-util-toc": "^4.0.0"
   },
   "devDependencies": {
     "browserify": "^16.0.0",

test/fixtures/slug-for-images-and-links/output.md

@@ -4,10 +4,10 @@
 
 -   [Something if](#something-if)
 
-    -   [Something else](#something-else)
-    -   [Something elsefi](#something-elsefi)
+    -   [![Something else](an-image.svg)](#something-else)
+    -   [![](an-alt.jpeg "Something elsefi")](#something-elsefi)
 
--   [another title](#another-title)
+-   [![Something iffi](a-link.png "a title")](#another-title)
 
 # [Something if](http://a-link.com)