Project refactor, add anthropic API support

- Split pkg/cli/cmd.go into new pkg/cmd package
- Split pkg/cli/functions.go into pkg/lmcli/tools package
- Refactor pkg/cli/openai.go to pkg/lmcli/provider/openai

Other changes:

- Made models configurable
- Slight config reorganization

pkg/cmd/prompt.go

@@ -0,0 +1,42 @@
+package cmd
+
+import (
+	"fmt"
+
+	cmdutil "git.mlow.ca/mlow/lmcli/pkg/cmd/util"
+	"git.mlow.ca/mlow/lmcli/pkg/lmcli"
+	"git.mlow.ca/mlow/lmcli/pkg/lmcli/model"
+	"github.com/spf13/cobra"
+)
+
+func PromptCmd(ctx *lmcli.Context) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "prompt [message]",
+		Short: "Do a one-shot prompt",
+		Long:  `Prompt the Large Language Model and get a response.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			message := inputFromArgsOrEditor(args, "# What would you like to say?\n", "")
+			if message == "" {
+				return fmt.Errorf("No message was provided.")
+			}
+
+			messages := []model.Message{
+				{
+					Role:            model.MessageRoleSystem,
+					Content: getSystemPrompt(ctx),
+				},
+				{
+					Role:            model.MessageRoleUser,
+					Content: message,
+				},
+			}
+
+			_, err := cmdutil.FetchAndShowCompletion(ctx, messages)
+			if err != nil {
+				return fmt.Errorf("Error fetching LLM response: %v", err)
+			}
+			return nil
+		},
+	}
+	return cmd
+}