Default the session socket to a named pipe on windows

[denusklo]

The session socket defaults to a path inside the user data directory, which
windows cannot bind: it has no Unix sockets. The bind failure is caught and
warned about, and the server is returned anyway, so startup prints

warn Could not create socket at ...\code-server-ipc.sock
info Session server listening on ...\code-server-ipc.sock

about a socket that does not exist, and nothing listens on it. That socket is
what shouldOpenInExistingInstance consults, so on windows code-server <file>
run from a shell can never find a running instance and starts its own server
instead.

Windows has the same facility under a different name, so this defaults to a
named pipe there. The name is derived from the user data directory, so
instances with separate data directories do not collide and a later invocation
with the same one finds the first, and it is case-folded first because windows
compares paths that way and the same directory typed two ways should not
produce two pipes. An explicitly passed --session-socket is untouched on every
platform.

Nothing downstream needed changing: the paths are only ever handed to
net.connect and http.request, both of which take a pipe name on windows, and
listen()'s unlink-first only has to tolerate ENOENT, which it already does.

Measured against a real windows build, one flag apart:

--session-socket '.\pipe...' no warning; "Session server listening on"
that pipe; canConnect true; GET /session
answers 200
(default) the warning above, the phantom "listening"
line, and no such file on disk

The helper takes the platform rather than reading process.platform directly,
following getEnvPaths and constructOpenOptions, so both branches are tested
where the tests run.