Modify md2html_hash_anchor_and_internal_link_filter to add an invisib…

[Zhaopudark]

To handle #3,

Modify md2html_hash_anchor_and_internal_link_filter to add an invisible link for each header.

• Now, for each header, an invisible link that links to the header itself will be added to it.
• The added invisible link will be assigned the class of headerlink.
• This will be useful for generating anchors by some renderers/frameworks/tools, such as hexo-theme-butterfly.

Note, all headers (<h1>~<h6>) will be inserted with an invisible header link.

Sometimes, in some renderers/frameworks/tools, the <h1> will be considered specially to form a title. But we hold the view that, in most cases, this special behavior does not conflict with this PR in terms of the visible effects. So, we make it influence all headers (<h1>~<h6>) for simplification, instead of ignoring <h1>.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.27%. Comparing base (62bac98) to head (7a33b12).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   93.23%   93.27%   +0.04%     
==========================================
  Files          22       22              
  Lines         458      461       +3     
==========================================
+ Hits          427      430       +3     
  Misses         31       31              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.