fix: render superscript/subscript as <sup>/<sub> instead of <em>

[morinokami]

Sätteri documents features.superscript / features.subscript as rendering ^text^ → <sup>text</sup> and ~text~ → <sub>text</sub>(Features docs). The options are accepted and the delimiters are consumed, but the output is wrong:

const source = '^text^ ~text~';

const { html } = markdownToHtml(source, {
  features: {
    superscript: true,
    subscript: true,
  },
});

console.log(html);
// expected: <p><sup>text</sup> <sub>text</sub></p>
// got: <p><em>text</em> <em>text</em></p>

Reproduction: https://stackblitz.com/edit/stackblitz-starters-mdzjlalh?file=index.js

This PR adds dedicated Superscript (33) and Subscript (34) MDAST node types.

Note

Same class of problem as #86 (headingAttributes): a documented feature flag that is accepted and consumes its syntax, but silently drops the result. This PR fixes the superscript/subscript case only; #86 remains open.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks
⏩ 8 skipped benchmarks¹

---

_{Comparing morinokami:fix-superscript-subscript (29e6c41) with main (efba0de)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Princesseuh]

Looks great, awesome work!