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.

provider/anthropic/anthropic.go

@@ -0,0 +1,11 @@
+// Package anthropic provides the Anthropic LLM provider.
+package anthropic
+
+import (
+	llm "gitea.stevedudenhoeffer.com/steve/go-llm"
+)
+
+// New creates a new Anthropic LLM provider with the given API key.
+func New(key string) llm.LLM {
+	return llm.Anthropic(key)
+}