Refactor: modularize and streamline LLM providers and utility functions

- Migrate `compress_image.go` to `internal/imageutil` for better encapsulation.
- Reorganize LLM provider implementations into distinct packages (`google`, `openai`, and `anthropic`).
- Replace `go_llm` package name with `llm`.
- Refactor internal APIs for improved clarity, including renaming `anthropic` to `anthropicImpl` and `google` to `googleImpl`.
- Add helper methods and restructure message handling for better separation of concerns.

functions.go

@@ -1,4 +1,4 @@
-package go_llm
+package llm
 
 import (
 	"reflect"