chore: Add Jest and testing-library

- Add jest
- Configure jest to work with [TypeScript][1] and [Gatsby][2]
- Add [testing-library][3] to help with unit-testing
- Refine storybook's file-glob search to match the new filename
  conventions

In order to remove some noise, components will follow a new name
convention, where each component will be be accompanied by
`stories.mdx` and `test.tsx` files.

TypeScript definitions for the testing-library matchers [are not used by
the compiler][4], unless we manually import the matchers on each test,
instead of using the `setupAfterEnvFiles` property from Jest.

[1]: https://kulshekhar.github.io/ts-jest/user/config/
[2]: https://www.gatsbyjs.org/docs/unit-testing/
[3]: https://testing-library.com/docs/dom-testing-library/example-intro
[4]: testing-library/jest-dom#11 (comment)

.storybook/config.js

@@ -19,4 +19,7 @@ window.___navigate = pathname => {
 addDecorator(withA11y);
 addDecorator(withKnobs);
 
-configure(require.context('../src', true, /\.stories\.(mdx|tsx)$/), module);
+configure(
+  require.context('../src', true, /(.*)(\.*)stories\.(js|ts|md)x*/),
+  module,
+);

jest.config.js

@@ -0,0 +1,20 @@
+module.exports = {
+  preset: 'ts-jest',
+  collectCoverage: true,
+  moduleNameMapper: {
+    '.+\\.(css|styl|less|sass|scss)$': `identity-obj-proxy`,
+    '.+\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$': `<rootDir>/support/file-mock.js`,
+  },
+  testPathIgnorePatterns: [
+    `node_modules`,
+    `.cache`,
+    '.storybook',
+    'coverage',
+    'public',
+    'storybook-static',
+  ],
+  setupFilesAfterEnv: ['@testing-library/jest-dom/extend-expect'],
+  globals: {
+    __PATH_PREFIX__: ``,
+  },
+};