feat(nodeenv): write the posix activate on Windows for git-bash

[ekalinin]

Closes #226.

On Windows nodeenv wrote only activate.bat, deactivate.bat and
Activate.ps1, so git-bash and the other posix shells available there had
nothing to source. Python's venv keeps its bash activate in
scripts/common/ and installs it on every platform; this does the same.

The script needed two Windows adjustments

• npm keeps the global modules next to node.exe, in Scripts/node_modules,
  not in lib/node_modules, and its prefix is the directory holding
  node.exe - so NODE_PATH and NPM_CONFIG_PREFIX point there.
• node.exe is a native binary and cannot read the /c/... paths a Windows
  shell hands out, so both variables are converted back with cygpath -w
  while $PATH stays posix, since that one is read by the shell itself.
  venv solves the same problem the same way.

The "do not call this file directly" guard now matches */Scripts/activate
too, and the $PATH line builds the .bin directory from __MOD_NAME__
instead of a hardcoded lib/node_modules - on posix both render exactly as
before.

Tests

tests/test_install_activate.py gets 8 tests that fake is_WIN, so the
Scripts/ layout is checked from any host: the set of files written, the
$PATH line, the native paths, sh -n over the generated script, the direct
call guard, and -p appending to the Scripts/activate that venv already
wrote there instead of replacing it.

Behaviour on a real Windows host is covered by test_smoke_git_bash, run by
a new git-bash CI job: shell: bash on a windows runner is git-bash. It
builds an environment, sources Scripts/activate and asserts that
process.execPath and npm root -g both point inside it - the second one
fails if the cygpath conversion did not happen.

Not in scope

• activate.fish is still posix-only, there is no native fish on Windows.
• The non-BASH_SOURCE fallback in ACTIVATE_SH still embeds the native
  C:\... path, which would break $PATH if a non-bash shell sourced it on
  Windows. git-bash ships bash as sh, so the case from the issue is covered.
• set_predeactivate_hook() writes .bat/.ps1 hooks only on Windows; a
  posix predeactivate for git-bash is a separate question.