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/gui/services/custom_commands/resolver.go

@@ -34,6 +34,11 @@ func (self *Resolver) resolvePrompt(
 		return nil, err
 	}
 
+	result.Body, err = resolveTemplate(prompt.Body)
+	if err != nil {
+		return nil, err
+	}
+
 	result.Command, err = resolveTemplate(prompt.Command)
 	if err != nil {
 		return nil, err