add TypeScript definition to task.js (#544)

Author: quisido

package.json

@@ -38,6 +38,7 @@
   },
   "homepage": "https://github.com/cypress-io/code-coverage#readme",
   "files": [
+    "*.d.ts",
     "*.js",
     "middleware"
   ],

task.d.ts

@@ -0,0 +1,6 @@
+/// <reference types="cypress" />
+
+export default function registerCodeCoverageTasks(
+  on: Cypress.PluginEvents,
+  config: Cypress.PluginConfigOptions,
+): void;