Large refactor - it compiles!

This refactor splits out all conversation concerns into a new
`conversation` package. There is now a split between `conversation` and
`api`s representation of `Message`, the latter storing the minimum
information required for interaction with LLM providers. There is
necessary conversation between the two when making LLM calls.

pkg/cmd/prompt.go

@@ -5,6 +5,7 @@ import (
 
 	"git.mlow.ca/mlow/lmcli/pkg/api"
 	cmdutil "git.mlow.ca/mlow/lmcli/pkg/cmd/util"
+	"git.mlow.ca/mlow/lmcli/pkg/conversation"
 	"git.mlow.ca/mlow/lmcli/pkg/lmcli"
 	"github.com/spf13/cobra"
 )
@@ -25,7 +26,7 @@ func PromptCmd(ctx *lmcli.Context) *cobra.Command {
 				return fmt.Errorf("No message was provided.")
 			}
 
-			messages := []api.Message{{
+			messages := []conversation.Message{{
 				Role:    api.MessageRoleUser,
 				Content: input,
 			}}