emscriptem bindings for root finding

[sverhoeven]

Adjust code blocks of https://github.com/NLESC-JCER/cpp2wasm#accessing-c-function-from-javascript-in-web-browser to Newton Raphson root finding algorithm.

Acceptance criteria:

• Someone (aka reviewer) is able to understand and run example
• Testing of chapter is automated

Tasks:

• Adapt code blocks
• Emscriptem installation instructions
• Command how to compile
• Include running example in Github Actions