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)

tests-ui/tests/litegraph/subgraph/SubgraphConversion.test.ts

@@ -1,9 +1,8 @@
 // TODO: Fix these tests after migration
 import { assert, describe, expect, it } from 'vitest'
 
+import type { ISlotType, LGraph } from '@/lib/litegraph/src/litegraph'
 import {
-  ISlotType,
-  LGraph,
   LGraphGroup,
   LGraphNode,
   LiteGraph