feat: add sum function

src/index.ts

@@ -1,2 +1,3 @@
 export * from "./greet.js";
+export * from "./sum.js";
 export * from "./types.js";

src/sum.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { sum } from "./sum.js";
+
+describe("sum", () => {
+	it("adds two numbers", () => {
+		expect(sum(1, 2)).toBe(3);
+	});
+
+	it("adds two negative numbers", () => {
+		expect(sum(-1, -2)).toBe(-3);
+	});
+
+	it("adds a positive and a negative number", () => {
+		expect(sum(1, -2)).toBe(-1);
+	});
+
+	it("adds two floating point numbers", () => {
+		expect(sum(1.1, 2.2)).toBeCloseTo(3.3);
+	});
+});

src/sum.ts

@@ -0,0 +1,3 @@
+export function sum(a: number, b: number): number {
+	return a + b;
+}