Reimplement `MarkdownProcessor` in Rust

[branyang02]

Rust (compiled to wasm) is faster, works with npm.

MarkdownProcessor in MarkdownProcessor.ts is essentially just a string processing step and outputs the preprocessed markdown string as well as equationMapping which is used for further downstream processes.

Requirements

New implementation should do exactly what MarkdownProcessor.ts does. Use an example markdown file to test the output of markdownContent.process() and compare.