Problem:

In WSL, explorer.exe should be checked if xdg-open isn't supported.

Solution:

I thought of using vim.ui.open(), which allows for more flexibility if the user wants to override it like so:

vim.ui.open = (function(overridden)
	return function(path, opt)
		-- logic
		return overridden(path, opt)
	end
end)(vim.ui.open)

this way it would have a uniform behaviour across all vim.ui.open() invocations, and it would reduce the code duplication, but
remembered that wouldn't work due to not being present in 0.8.*.

So updated the code to resemble the upstream vim.ui.open() implementation, increasing handler support.