[kbn-scout] initial package implementation (playwright-test POC) (#194006)

## Summary

This PR introduces a new package, **kbn-scout**, designed to streamline
the setup and execution of Playwright tests for Kibana. The `kbn-scout`
package consolidates server management and testing capabilities by
wrapping both the Kibana/Elasticsearch server launcher and the
Playwright test runner. It includes:

- Test and worker-scoped fixtures for reliable setup across test suites
- Page objects combined into the fixture for Kibana UI interactions
- Configurations for seamless test execution in both local and CI
environments (`Cloud` execution is out of scope)
- This package aims to simplify test setup and enhance modularity,
making it easier to create, run, and maintain deployment-agnostic tests,
that are located in the plugin they actually test.

Tests example is available in `x-pack/plugins/discover_enhanced` plugin
under `ui_tests` folder

How to run:
1) As a single script (servers + tests): 
```
node scripts/scout_test.js --config=x-pack/plugins/discover_enhanced/ui_tests/playwright.config.ts --serverless=es
```

2) Start servers first
```
node scripts/scout_start_servers.js --serverless=es
```
then run tests:
```
npx playwright test --config=x-pack/plugins/discover_enhanced/ui_tests/playwright.config.ts
```
if you have Playwright plugin in IDEA, just use it to run tests files

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

packages/kbn-repo-source-classifier/src/config.ts

@@ -58,5 +58,6 @@ export const TEST_DIR = new Set([
   'storybook',
   '.storybook',
   'integration_tests',
+  'ui_tests',
   ...RANDOM_TEST_FILE_NAMES,
 ]);