[jtulach]

• this PR proposes a refactoring of the jshell.support module's ShellSession, however ...
• my primary goal is elsewhere:
  • I have a JShell instance
  • I'd like to connect such an instance to the existing NetBeans JShell editing functionality
  • however that functionality seems to demand to create its own JShell instance
  • I am searching for the smoothest way to untangle the creation from the rest of editing functionality
  • @lahodaj, @sdedic your guidance is welcomed to help me achieve such a goal!
• previous ShellSession mixed API and implementation
  • cleaning that up was thus my first idea
  • this PR is result of such a cleanup
  • it moves the implementation into ShellSessionImpl
• the rest of the module is supposed to call only the ShellSession API class
  • only ShellEnvironment instantiates and deals directly with ShellSessionImpl
• such a refactoring shall be harmless and potentially useful for the goal of attaching existing JShell instance