Add DeepSeek support (#23551)

- Added support for DeepSeek as a new language model provider in Zed
Assistant
- Implemented streaming API support for real-time responses from
DeepSeek models.
- Added a configuration UI for DeepSeek API key management and settings.
- Updated documentation with detailed setup instructions for DeepSeek
integration.
- Added DeepSeek-specific icons and model definitions for seamless
integration into the Zed UI.
- Integrated DeepSeek into the language model registry, making it
available alongside other providers like OpenAI and Anthropic.

Release Notes:

- Added support for DeepSeek to the Assistant.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

assets/settings/default.json

@@ -1166,6 +1166,9 @@
     },
     "lmstudio": {
       "api_url": "http://localhost:1234/api/v0"
+    },
+    "deepseek": {
+      "api_url": "https://api.deepseek.com"
     }
   },
   // Zed's Prettier integration settings.