Close stray process when stdio pipe is closed or possibly when parent process is killed (daemon flag?)

This is continuation of the effort from:

1. Issue https://github.com/PowerShell/PowerShellEditorServices/issues/655 PSES needs to shutdown itself when it's supposed to
2. PR https://github.com/PowerShell/PowerShellEditorServices/pull/663 Close stray processes on exit

#655 was closed by @TylerLeonhardt as it was satisfied by PR #663. However I still believe we can easily get into the situation when LSP text editor that launches PSES gets killed and orphans the PSES process, that keeps running without a purpose. Actually this is the case with Sublime Text and LSP plugin, where other language LSP providers shutdown themselves when the Sublime Text quits without a need to use exit or any other means.

Therefore I propose 2 options for consideration (alternatives):

1. Introduce new -daemon flag, that unless specified, makes PSES to shutdown itself when the parent process disappears. This will make PSES behave like most of the command line tools, i.e. shutdown itself when the shell or launching process quits.

   The only purpose for PSES to run despite of parent process being alive or not is when it is it intended to be ran as a daemon, and I believe this intent should not be default (as it is now).

2. Shutdown itself when running as -stdio and when pipe is closed, as I was commenting in other ticket, there is no point running when pipe is closes, because there is nobody that will be able to issue exit and any other command anyways.

Thank you in advance for considering my suggestion. If you prefer I can try to get this behavior adapter myself and send the PR, but you (maintainers) may come to more elegant solution much quicker than me, therefore I post this issue first.

Prüfe zuerst issue #655 und PR #663 und verfolge anschließend die PSES-Einstiegspunkte für den -stdio-Modus und das vorgeschlagene -daemon-Flag. Entscheide, welches Shutdown-Verhalten vorgesehen ist, wenn der Parent verschwindet oder die stdio-Pipe geschlossen wird; erledigt ist die Aufgabe, wenn im abgedeckten Szenario keine verwaisten PSES-Prozesse mehr verbleiben, ohne die Verwendung als Daemon zu beeinträchtigen.