merge .gitignore when we merge emscripten branch with master

There's a separate .gitignore in emscripten/projects/purr-data that only applies for that subdirectory for building using emscripten.

This works right now since it doesn't apply to anything but the emscripten branch which is not yet in master. But when we merge emscripten branch with master, we'll want to merge the two .gitignore files. While git technically allows to have multiple ones it's more readable and maintainable to just have the one in the root directory.