feat: Add type definitions (#16)

Fixes #4

cypress/integration/spec.js

@@ -1,4 +1,5 @@
 /// <reference types="cypress" />
+/// <reference path="../../src/index.d.ts" />
 
 describe('cypress-xpath', () => {
   it('adds xpath command', () => {

src/index.d.ts

@@ -0,0 +1,15 @@
+/// <reference types="cypress" />
+
+declare namespace Cypress {
+  interface Chainable<Subject> {
+    /**
+    * Get one or more DOM elements by an XPath selector.
+    * **Note:** you can test XPath expressions from DevTools console using $x(...) function, for example $x('//div') to find all divs.
+    * @see https://github.com/cypress-io/cypress-xpath
+    * @example
+    * cy.xpath(`//ul[@class="todo-list"]//li`)
+    *   .should('have.length', 3)
+    */
+   xpath<E extends Node = HTMLElement>(selector: string, options?: Partial<Loggable & Timeoutable>): Chainable<JQuery<E>>
+  }
+}