Extension should not duplicate integrated console terminals

[Tyriar]

You can get some context on this here: microsoft/vscode#123805

Basically PS integrated consoles get reconnected when the window is reloaded currently by design, the PS extension should detect these and kill them before trying to start up another one (or even better re-establish the connection if that's possible).

[TylerLeonhardt]

@rjmholt @andschwa You should be able to get all terminals with name "PowerShell Integrated Console" and kill them before creating the one the new session will use... but I thought I saw this bug so just heads up if you do (and report back if you do!):
microsoft/vscode#123805 (comment)

[andyleejordan]

Wee! I tested out the proposed API and it works great. Just waiting for it to be available in a release 😄