net: add createPipe()

[kingces95]

Adds a node:pipe module with createPipe(), returning a readable
and writable endpoint owned by the parent process.

The endpoints may be passed to child_process.spawn() stdio. This lets
the parent lend a pipe endpoint to a child without turning the parent
stream itself into child-owned stdio. The parent can then reclaim unread
bytes or lend the same endpoint to a later child.

This is useful for bash-like partial consumption of long-lived streams.
For example, a parent can keep ownership of an Server Sent Event stream
while delegating bounded reads to external tools, then continue parsing
from the exact byte where the child stopped.

The central integration test writes abc, lets child A read a, lets
child B read b, and then verifies that the parent can still read c.

Endpoints created by pipe.createPipe() are rejected by spawnSync(), and
an endpoint may only be leased to one child process at a time.

Tests cover node:pipe creation, documentation examples, child stdio
leasing, sequential lease-and-reclaim behavior, lease error cleanup,
spawnSync() rejection, and platform-consistent writable finish
behavior.