Add suitable default open statement for FreeBSD

[kMutagene]

Chart.show uses an OS-specific way of opening files:

Plotly.NET/src/Plotly.NET/InternalUtils.fs

Lines 93 to 105 in 2f94e87

	///Choose process to open plots with depending on OS. Thanks to @zyzhu for hinting at a solution (https://github.com/plotly/Plotly.NET/issues/31)
	let openOsSpecificFile path =
	if RuntimeInformation.IsOSPlatform(OSPlatform.Windows) then
	let psi =
	new System.Diagnostics.ProcessStartInfo(FileName = path, UseShellExecute = true)
	System.Diagnostics.Process.Start(psi) |> ignore
	elif RuntimeInformation.IsOSPlatform(OSPlatform.Linux) then
	System.Diagnostics.Process.Start("xdg-open", path) |> ignore
	elif RuntimeInformation.IsOSPlatform(OSPlatform.OSX) then
	System.Diagnostics.Process.Start("open", path) |> ignore
	else
	invalidOp "Not supported OS platform"

Since we do not have a match case for FreeBSD, you currently cannot display charts via Chart.show on that OS.

Since i am not familiar with that OS, I would need input from someone that actually uses it on how to proceed here.

My first google attempt comes up with this, but i have no way of testing it.

See also #384