Emscripten: ofLaunchBrowser implementation

[artificiel]

Allows one to use ofLaunchBrowser() in emscripten. Added an optional parameter to ofLaunchBrowser that passes the target parameter of the window.open() call. defaults to _blank for a fresh page.

closes #7591

[Jonathhhan]

Nice, thanks :)

[ofTheo]

Thanks!
I think this would need the target argument to be implemented for non emscripten platforms before we merge.

[artificiel]

@ofTheo no rush for that!

there's a bit of a design decision to make as the OS URL pathways don't allow the same target feature (at least on macOS and linux). so I don't know how it could be usefully used?

if we want to unpollute the API of desktop we could have a specifc function defined on emscripten, splitting the top part of ofLaunchBrowser so it can be reused.