Start work on porting kitty shell to Go

tools/cmd/at/shell.go

@@ -0,0 +1,49 @@
+// License: GPLv3 Copyright: 2022, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package at
+
+import (
+	"fmt"
+
+	"kitty/tools/cli"
+	"kitty/tools/cli/markup"
+	"kitty/tools/tui/loop"
+)
+
+var _ = fmt.Print
+
+var formatter *markup.Context
+
+const prompt = "🐱 "
+
+func shell_loop(kill_if_signaled bool) (int, error) {
+	lp, err := loop.New(loop.NoAlternateScreen, loop.NoRestoreColors)
+	if err != nil {
+		return 1, err
+	}
+	lp.OnInitialize = func() (string, error) {
+		lp.QueueWriteString(prompt)
+		return "\r\n", nil
+	}
+
+	err = lp.Run()
+	if err != nil {
+		return 1, err
+	}
+	ds := lp.DeathSignalName()
+	if ds != "" {
+		if kill_if_signaled {
+			lp.KillIfSignalled()
+			return 1, nil
+		}
+		return 1, fmt.Errorf("Killed by signal: %s", ds)
+	}
+	return 0, nil
+}
+
+func shell_main(cmd *cli.Command, args []string) (int, error) {
+	formatter = markup.New(true)
+	fmt.Println("Welcome to the kitty shell!")
+	fmt.Println("Use", formatter.Green("help"), "for assistance or", formatter.Green("exit"), "to quit.")
+	return shell_loop(true)
+}