lint: add tsconfig for browser_tests, fix existing violations (#5633)

## Summary

See https://typescript-eslint.io/blog/project-service/ for context.
Creates a browser_tests specific tsconfig so that they can be linted.

Does not add a package.json script to do the linting yet, but `pnpm exec
eslint browser_tests` should work for now.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5633-lint-add-tsconfig-for-browser_tests-fix-existing-violations-2726d73d3650819d8ef2c4b0abc31e14)
by [Unito](https://www.unito.io)

browser_tests/tsconfig.json

@@ -0,0 +1,14 @@
+{
+  "extends": "../tsconfig.json",
+  "compilerOptions": {
+    /* Test files should not be compiled */
+    "noEmit": true,
+    // "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "resolveJsonModule": true
+  },
+  "include": [
+    "**/*.ts",
+  ]
+}