Reimplement `MarkdownProcessor` in Rust

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.