How to find the default webbrowser on Windows and Mac?

General Usage
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In the help menu of our app we do:

url = "https://albatross-kite-transport.github.io/VortexStepMethod.jl/dev"
run(`xdg-open $url`)

to open the default web browser with the given URL.

This works on Linux, but how can this be done on Windows and Mac?

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