feat: Add LaTeX and markdown extensions

[Fuyeors]

Summary

Extend the existing LaTeX parser work with five Interactify-compatible Markdown extensions:

• Add Twemoji AST transformation for Unicode emoji while preserving code spans and code blocks.
• Add highlight.js-compatible fenced-code output, including language alias normalization for JavaScript, TypeScript, FFM, FON, FER, and other common aliases.
• Add Mermaid and ABC fenced-code placeholders for asynchronous frontend rendering.
• Add fenced and inline SMILES placeholders, including the FFM syntax #[smiles = SMILES_CODE].
• Add a lazy AST traversal utility and a combined extensionsPlugin so the default FFM parser performs one post-parse extension pass.

The existing latexPlugin remains available independently. createFuyeorMarkdownParser() enables LaTeX and the five new extensions by default, while individual plugins remain publicly exportable for opt-in parser configurations.

Rendering contract

The implementation follows the existing Interactify asynchronous processor contract:

• Emoji: <img class="emoji" draggable="false" alt="..." src="..."/>
• Mermaid: <div class="language-mermaid">...</div>
• ABC: <div class="language-abc">...</div>
• Fenced SMILES: one <div class="language-smiles smiles-block" data-smiles="..."></div> per non-empty line
• Inline SMILES: <span class="language-smiles smiles-inline" data-smiles="..."></span>
• Highlightable code: pre.hljs.language-${lang}[data-language="${lang}"]

All user-controlled content inserted into HTML attributes or text placeholders is escaped.

Validation

Check	Result
TypeScript typecheck	Pass
Parser unit tests	21/21 pass
FFM regression tests	10 pass, 458 skipped by existing filters
Prettier check	Pass
git diff --check	Pass

Performance

Five-run local benchmarks with identical Node/Vitest settings showed that plain text without extension markers changed from a median of 2.185 ms to 2.204 ms per iteration (+0.9%) after the lazy single-pass optimization. New extension paths are measured separately because the baseline parser emitted ordinary text/code rather than the requested structured placeholders. Temporary benchmark files were not included in the commit.

Package size

Compared with the existing LaTeX branch using npm pack, the tarball increased from 36,278 B to 40,154 B (+3,876 B, +10.7%), while unpacked size increased from 212,359 B to 226,779 B (+14,420 B, +6.8%). The increase is source-only and includes the five extension modules, AST traversal helper, and regression tests; no runtime dependency was added.