Improve JS and HTML engine run invokations: Allow running the default system browser on run without having to set environment variables. Add support for detecting EMSCRIPTEN_BROWSER and EMSCRIPTEN_JSENGINE environment variables if user wants to not use the default system browser, but some other.
Fix bad string slash conversion that occurred on -D directives, e.g. -DSOMESTRING=\"this is a string\" would get passed as -DSOMESTRING=/"this is a string/" to emcc, which would not properly receive the quotes.