[scout] fix playwright configs discovery script and add save flag for CI

[dmlemeshko]

Summary

This PR fixes the search logic to look for playwright configs in all possible & expected locations (src/* was not working), matching one of 3 regexp:

      /(x-pack\/platform\/plugins\/(?:private|shared|[^\/]+)\/([^\/]+))\/ui_tests\//,
      /(x-pack\/solutions\/[^\/]+\/plugins\/([^\/]+))\/ui_tests\//,
      /(src\/platform\/plugins\/(?:private|shared)?\/?([^\/]+))\/ui_tests\//,

For each plugin we also have usesParallelWorkers prop (true if at least 1 config runs with concurrent workers) to decide later, if we need worker with 4 or 8 VCPUs.

The idea is to run node scripts/scout discover-playwright-configs --save on CI and use generated json as source to build test run pipeline.

Current output:

{
  "discover_enhanced": {
    "group": "platform",
    "pluginPath": "x-pack/platform/plugins/private/discover_enhanced",
    "configs": [
      "x-pack/platform/plugins/private/discover_enhanced/ui_tests/parallel.playwright.config.ts",
      "x-pack/platform/plugins/private/discover_enhanced/ui_tests/playwright.config.ts"
    ],
    "usesParallelWorkers": true
  },
  "maps": {
    "group": "platform",
    "pluginPath": "x-pack/platform/plugins/shared/maps",
    "configs": [
      "x-pack/platform/plugins/shared/maps/ui_tests/playwright.config.ts"
    ],
    "usesParallelWorkers": false
  },
  "observability_onboarding": {
    "group": "observability",
    "pluginPath": "x-pack/solutions/observability/plugins/observability_onboarding",
    "configs": [
      "x-pack/solutions/observability/plugins/observability_onboarding/ui_tests/parallel.playwright.config.ts",
      "x-pack/solutions/observability/plugins/observability_onboarding/ui_tests/playwright.config.ts"
    ],
    "usesParallelWorkers": true
  }
}

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 756bd1e

Metrics [docs]

✅ unchanged

cc @dmlemeshko