feat: allow disabling codebase indexing

[elasticdotventures]

This PR introduces the ability to disable the codebase indexing feature via VS Code settings. It addresses the oversight where the indexing feature, once enabled, could not be easily turned off.\n\nChanges include:\n- Modifying to read from user configuration and update accordingly.\n- Exposing in and for user control.\n- Updating and to correctly handle the new setting.\n- Modifying to gracefully handle disabled or unconfigured indexing by returning empty results instead of throwing errors.\n\nCloses #5623

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Important

Adds a setting to enable or disable codebase indexing in VS Code, affecting semantic search availability.

• Settings:
  • Adds settings.codebaseIndexing.enabled.description to package.nls.json to allow enabling or disabling codebase indexing.
• Behavior:
  • When codebase indexing is disabled, semantic search is unavailable.
  • Handles disabled or unconfigured indexing by returning empty results instead of errors.

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[daniel-lxs]

Thanks for the contribution. I'm already working on this in PR #5599.

Apologies for the overlap, and we really appreciate you taking the time to look into it.