feat: add confirm prompt for custom keybindings

- Supports configuring a custom confirmation prompt via `config.yml` for
  custom keybindings. A new `CustomCommandPrompt.Body` field is
  used to store the immutable body text of the confirmation popup.
- Adds a sample 'confirm' prompt to the example `config.yml`.
- Updates the `Prompts` section of the documentation to include
  'confirm' prompt type and also describe which fields pertain to it
  (i.e. `initialValue`).

Closes: https://github.com/jesseduffield/lazygit/issues/1858

Signed-off-by: Michael Mead <mmead.developer@gmail.com>

pkg/config/user_config.go

@@ -313,12 +313,15 @@ type CustomCommand struct {
 }
 
 type CustomCommandPrompt struct {
-	Type  string `yaml:"type"` // one of 'input' and 'menu'
+	Type  string `yaml:"type"` // one of 'input', 'menu', or 'confirm'
 	Title string `yaml:"title"`
 
 	// this only apply to prompts
 	InitialValue string `yaml:"initialValue"`
 
+	// this only applies to confirm prompts
+	Body string `yaml:"body"`
+
 	// this only applies to menus
 	Options []CustomCommandMenuOption