Add superscript and subscript span extensions

[hryhoriiK97]

Summary

This PR adds two opt-in inline span extensions to MD4C.

The new syntaxes are:

x^2^    (superscript)
H~2~O   (subscript)

When MD_FLAG_SUPERSCRIPTS is enabled, MD4C emits a new MD_SPAN_SUPERSCRIPT
span. When MD_FLAG_SUBSCRIPTS is enabled, MD4C emits a new MD_SPAN_SUBSCRIPT
span. The bundled HTML renderer outputs them as:

x<sup>2</sup>
H<sub>2</sub>O

md2html gets two new options: --fsuperscripts and --fsubscripts.

When MD_FLAG_SUBSCRIPTS and MD_FLAG_STRIKETHROUGH are both active, ~text~
renders as subscript and ~~text~~ remains strikethrough, preserving GFM
compatibility.

A 600-second OSS-Fuzz run found no crashes or assertion failures.

These extensions will be used in react-native-enriched-markdown.

[codecov]

Codecov Report

❌ Patch coverage is 83.09859% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (347b528) to head (5baf679).

Files with missing lines	Patch %	Lines
src/md4c.c	81.53%	0 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   78.94%   78.98%   +0.03%     
==========================================
  Files           5        5              
  Lines        3401     3464      +63     
  Branches     1138     1167      +29     
==========================================
+ Hits         2685     2736      +51     
  Misses        275      275              
- Partials      441      453      +12     

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[mity]

Rebased and merged into master branch manually.