Configure ESLint's environment for Jest's global functions

Since Jest makes use of the globally provided functions, ESLint's
environment must be configured to know that Jest is used in the project.

GH-39

.eslintrc.js

@@ -13,6 +13,7 @@
  * @author Sven Greb <[email protected]>
  * @see https://github.com/babel/eslint-plugin-babel#rules
  * @see https://github.com/tc39/proposal-optional-chaining
+ * @see https://eslint.org/docs/user-guide/configuring#specifying-environments
  */
 
 const { resolve } = require("path");
@@ -23,6 +24,7 @@ module.exports = {
   parser: "babel-eslint",
   env: {
     browser: true,
+    jest: true,
     node: true
   },
   settings: {