First approach to fix issue with dbg/run start before PSES running

[rkeithhill]

PR Summary

Fix #1433

One minor issue is that if you abort (return null|undefined) from
resolveDebugConfiguration, VSCode "helpfully" opens your launch.json
file for you. Actually, that is quite annoying. I found an issue and on
this and voted it up - microsoft/vscode#54213

Also fix logic for "attach" error. We only need to test if
OS != Windows. If on Windows, PS Core supports attach. And tweaked the
error message wording to make more clear.

If the user attempts to start a dgb or "run with out debugging" session
before PSES is running, a NullRefEx occurs in PSES. Ideally, we would
wait in the resolveDebugConfiguration method for PSES to finish
initializing with a max wait time of say 10 seconds. Unfortunately,
"sleep" in a loop in JavaScript is not so easy. AFAIT requires a
significant rewrite of the method using setTimeout(). Not sure it is
worth it, unless someone more knowledgable in JS can find an easy
way to do the poll (for sessionstatus)/sleep loop.

BTW there is probably a fix we need to make in PSES to check if
SynchronizationContext is not null before we try to use it.

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[TylerLeonhardt]

I take it you're able to start debugging, get the warning, then wait, then start debugging again, and it works?

[rkeithhill]

Yes.

[TylerLeonhardt]

LGTM :)

[rjmholt]

This looks good to me! I think that just firing the warning instead of waiting until the the server is ready probably makes sense -- although if we really don't like it we can always work on that as a longer term solution.