add claude 4 sonnet and claude 4 opus (#137)

Author: kristencheung

model-router.mdx

@@ -694,6 +694,8 @@ Large language models provide text generation and reasoning capabilities.
 
 | Provider  | Model                               | Slug                                        |
 | --------- | ----------------------------------- | ------------------------------------------- |
+| Anthropic | Claude 4 Sonnet                     | `claude-sonnet-4-20250514`                  |
+| Anthropic | Claude 4 Opus                       | `claude-opus-4-20250514`                    |
 | Anthropic | Claude 3.7 Sonnet (latest)          | `claude-3-7-sonnet-latest`                  |
 | Anthropic | Claude 3.7 Sonnet                   | `claude-3-7-sonnet-20250219`                |
 | Anthropic | Claude 3.5 Sonnet (latest)          | `claude-3-5-sonnet-latest`                  |