feat(calculator.ts): add cos method

Author: gfmio

src/Calculator.ts

@@ -61,4 +61,9 @@ export default class Calculator {
     this.current = Math.sin(this.current);
     return this;
   }
+
+  public cos() {
+    this.current = Math.cos(this.current);
+    return this;
+  }
 }

tests/Calculator.test.ts

@@ -17,6 +17,7 @@ describe("Calculator", () => {
 
   it("can clear", () => expect(calculator.clear().value()).equal(0));
   it("can sin", () => expect(calculator.sin().value()).equal(0));
+  it("can cos", () => expect(calculator.cos().value()).equal(1));
 
   it("has the correct initial value when no argument is supplied", () => expect(new Calculator().value()).equal(0));
   it("has the correct initial value when an argument is supplied", () => expect(new Calculator(3).value()).equal(3));