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/composables/canvas/useSelectedLiteGraphItems.test.ts

@@ -2,12 +2,8 @@ import { createPinia, setActivePinia } from 'pinia'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import { useSelectedLiteGraphItems } from '@/composables/canvas/useSelectedLiteGraphItems'
-import {
-  LGraphEventMode,
-  LGraphNode,
-  Positionable,
-  Reroute
-} from '@/lib/litegraph/src/litegraph'
+import type { LGraphNode, Positionable } from '@/lib/litegraph/src/litegraph'
+import { LGraphEventMode, Reroute } from '@/lib/litegraph/src/litegraph'
 import { useCanvasStore } from '@/renderer/core/canvas/canvasStore'
 import { app } from '@/scripts/app'