Don't kill process when embedded

[fmagin]

This tries is intended as a fix for GhidraJupyter/ghidra-jupyter-kotlin#2

Using exitProcess(0) will terminate the entire process, but if the kernel is used as a library and embedded in another application, i.e. process this will be terminated too. Instead the code that handles the ShutdownRequest now has a somewhat inelegant check if the kernel is embedded and will instead just throw an InterruptedException which is caught in the controlThread loop and leads to the controlThread interrupting the mainThread so they both terminate and the entire kernel terminates.

Because I don't know why exitProcess(0) was used in the first place this will only happen if the kernel is embedded for now, but if this also works for the regular usage this could be simplified to just always throwing an InterruptedException