Add support for persistent macros between renders

[dahlia]

This patch adds support for persisting user-defined macros between renders in markdown-it-texmath.

To persist macros, pass an env object to the .render() method:

const macros = { "\\RR": "\\mathbb{R}" };
texmath.render(input, { macros });

Macros defined during rendering will update the macros object, allowing reuse in subsequent renders.