Add multiplication functionality with tests

[Copilot]

Implements basic math utilities starting with multiplication, based on reference commit 8d022b2.

Changes

• src/math.js: Exports multiply(a, b) function
• test/math.test.js: Test suite covering positive/negative numbers, zero handling, and boundary values
• package.json: Node.js project setup with mocha testing framework

import { multiply } from './src/math.js';

multiply(2, 3);      // 6
multiply(-2, 3);     // -6
multiply(0, 5);      // 0

All 4 tests passing.

Original prompt

Reference: 8d022b2

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