fix: preserve host process.exitCode when closing PGlite

[zakcutner]

Closing a PGlite instance resets the host process's process.exitCode to 0. This wipes out whatever the host had already set.

The cause is close() calling _emscripten_force_exit(0). Emscripten implements exit as quit_ = (e, r) => { throw process.exitCode = e, r }, so it writes process.exitCode = 0 on the host Node process. The #init() and execProtocolRaw() paths already save and restore process.exitCode around their Emscripten calls, but close() was missing the same guard.

This adds that guard. It reuses the existing pglUtils.pgliteProc.exitCode save and restore pattern in a finally, like execProtocolRaw(). It also adds a test next to the existing restores process.exitCode one, plus a changeset.