fix: Escaping of link labels (particularly if link label is an URL).

Previously an URL at the syntactic position of a link label could
result in the colons after the URL proctocol scheme being escaped
by a backslash:
~~~
[https\://github.com//issues/122][1]
~~~

An update of mdast-util-to-markdown (0.6.1 -> 0.6.2) fixes this.
New output is:
~~~
[#122]
~~~