[Feature Request]: Code coverage for Emscripten systems

[mcbarton]

Description of new feature

Currently we only have a coverage number for a native build. In future we should be able to have a code coverage percentage for the Emscripten build. Emscripten doesn't currently support coverage. By keeping track of emscripten-core/emscripten#13046 , and the links within, will give an idea at which point CppInterOp can support have an Emscripten coverage percentage.