Add option to completely disable notifications from Coder

[blinkagent]

Summary

Add a setting that allows users to completely disable notifications from the Coder extension (e.g., the prompt shown to update a workspace when it's outdated).

Motivation

Some users find the notifications disruptive and want the ability to suppress them entirely.

Proposed Approaches

1. Ignore messages from the WebSocket — Continue to maintain the WS connection but silently discard notification-triggering messages.
2. Disconnect the WebSocket (and reconnect when re-enabled) — Fully disconnect from the WS when notifications are disabled, and re-establish the connection when the user re-enables them.

Acceptance Criteria

• A new user-facing setting (e.g., coder.disableNotifications) that controls whether notifications are shown.
• When disabled, no notification prompts (workspace outdated, etc.) should appear.
• Re-enabling the setting should restore normal notification behavior without requiring a reload.

---

Created on behalf of @EhabY