feat: add MockBinding.connect virtual loop cable

[JSap0914]

Implements the "virtual loop cable" feature requested in serialport/node-serialport#1580 (good first issue).

What

Adds a MockBinding.connect(path1, path2) static method that wires two mock ports together with a virtual loop cable, like a physical null-modem/loopback cable between two serial ports. Data written to one connected port is delivered to the other port's read buffer (and vice versa), instead of being echoed back to the writer.

MockBinding.createPort('/dev/portA')
MockBinding.createPort('/dev/portB')
MockBinding.connect('/dev/portA', '/dev/portB')

const a = await MockBinding.open({ path: '/dev/portA', baudRate: 9600 })
const b = await MockBinding.open({ path: '/dev/portB', baudRate: 9600 })

await a.write(Buffer.from('hello'))
// `b` can now read "hello"

Behavior

• Bidirectional: writes on either end are readable on the other.
• Wakes a read that is already pending on the peer.
• No self-echo: the writer does not receive its own data (distinct from the existing echo option).
• Data written while the peer is closed is dropped (nothing is buffered for a disconnected receiver).
• connect throws if either port has not been created first.

Tests

Adds regression tests for delivery, bidirectionality, waking a pending peer read, dropped writes when the peer is closed, and both-ports-exist validation. Full suite (npm test) passes (12 passing); npm run lint and npm run build are clean.

Docs

README updated with a "Virtual loop cable" section.

Closes serialport/node-serialport#1580