chore: Add linter for typescript code (#45)

- Add and configure `eslint`
- Add to build pipeline
- Fix lint failures

jest-runner.eslint.config.js

@@ -0,0 +1,13 @@
+// Toggle eslint --fix by specifying the `FIX` env.
+const fix = !!process.env.FIX
+
+module.exports = {
+  cliOptions: {
+    ext: [".js", ".ts", ".tsx"],
+    ignorePath: ".eslintignore",
+    cache: false,
+    fix,
+    resolvePluginsRelativeTo: ".",
+    maxWarnings: 0,
+  },
+}