fix(nodeenv): keep the python virtualenv's scripts under -p on Windows

[ekalinin]

Fixes #243.

The problem

nodeenv -p inside a python virtualenv on Windows overwrote
Scripts/activate.bat, Scripts/deactivate.bat and
Scripts/Activate.ps1, so VIRTUAL_ENV and the virtualenv's own
deactivation were gone after the run. Only the posix Scripts/activate
survived, because that one is appended.

install_activate() picked append over overwrite by
bool(disable_prompt), and DISABLE_PROMPT has entries only for
activate and activate.fish, never for the cmd and PowerShell
scripts.

The fix

• the choice now goes by the file: everything python's virtualenv
  writes is extended, listed in PYTHON_VIRTUALENV_FILES. For posix
  this is the same set as before, and shim/node are still
  overwritten
• ACTIVATE_PS1 defines deactivate as well, and so does the
  virtualenv's own Activate.ps1 it is appended to. It now saves the
  one already in place as _OLD_NODE_DEACTIVATE and puts it back and
  calls it on a real deactivate. The internal deactivate -nondestructive does not chain, or it would undo the activation that
  just happened above it
• writefile() starts an appended part on a line of its own: a
  deactivate.bat ending with :END and no trailing newline swallows
  the first appended line into the label

ACTIVATE_BAT and DEACTIVATE_BAT are unchanged. Under -p both
environments share one directory, so the _OLD_VIRTUAL_PROMPT /
_OLD_VIRTUAL_PATH bookkeeping the two scripts have in common composes
correctly: PATH and PROMPT after activation are the ones the virtualenv
set, and on deactivation the virtualenv's part runs first and clears
them, so the appended part leaves them alone.

Tests

• tests/test_install_activate.py: the three cmd/PowerShell scripts
  keep the content the virtualenv wrote, and Activate.ps1 keeps a
  deactivate that is already there
• tests/nodeenv_test.py: writefile() append starts on a new line
• tests/test_activate_win.py: new, the counterpart of
  test_activate_shells.py for cmd and PowerShell. It runs the scripts
  for real and checks VIRTUAL_ENV, NODE_VIRTUAL_ENV and PATH after
  activation and after deactivation. Needs a Windows host, so it only
  runs in the windows-latest jobs