feat: use md2html instead of reverse-engineering the DOM

[kptdobe]

The current implementation of the view doc source feature uses the HTML rendition of the page and revere-engineers the source document. This was initially not a good idea because it cannot be fully precise, for example accurately computing the metadata is impossible (metadata.xslx rules vs computed metadata vs metadata block).

Using the md rendition and show it as html is the point of this PR.

[codecov]

Codecov Report

Merging #189 (3aa200c) into main (f5e02aa) will decrease coverage by 11.60%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

❗ Current head 3aa200c differs from pull request most recent head 1ad0944. Consider uploading reports for the commit 1ad0944 to get more accurate results

@@             Coverage Diff             @@
##             main     #189       +/-   ##
===========================================
- Coverage   89.94%   78.34%   -11.60%     
===========================================
  Files           5        8        +3     
  Lines        5150     4304      -846     
===========================================
- Hits         4632     3372     -1260     
- Misses        518      932      +414     

Files	Coverage Δ
src/extension/view-doc-source/js/blocks.js	95.58% <ø> (-4.42%)	⬇️

... and 8 files with indirect coverage changes

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