feat(calculator.ts): add inverse method

Author: gfmio

src/Calculator.ts

@@ -71,4 +71,9 @@ export default class Calculator {
     this.current = Math.tan(this.current);
     return this;
   }
+
+  public inverse() {
+    this.current = div(1, this.current);
+    return this;
+  }
 }

tests/Calculator.test.ts

@@ -22,4 +22,6 @@ describe("Calculator", () => {
   it("can cos", () => expect(calculator.cos().value()).equal(1));
   // prettier-ignore
   it("can tan", () => expect(calculator.clear().tan().value()).equal(0));
+  // prettier-ignore
+  it("can invert", () => expect(calculator.clear().add(2).inverse().value()).equal(1 / 2));
 });