C++-demangle JS function names for emscripten'd code

If you compile a C++ project to JS using emscripten, you end up with JS function names like `__ZN20ShaderConstantBuffer9SetFloat3EiRKN4math6float3E`. Profiles of such code can be hard to read. We should run function names through a demangler so that they become readable; in the example that would be `ShaderConstantBuffer::SetFloat3(int, math::float3 const&)`.

There's a `demangle` module on npm that can do this.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-117)