`show_default` as a string isn't used in prompts as the default shown

[thejcannon]

The behavior of click.Option("--name", default="default", show_default="show_default", prompt=True) is different for the help text and for the prompt.

Help: 👍

Usage: foo.py [OPTIONS]

Options:
  --name TEXT  [default: (show_default)]
  --help       Show this message and exit.

Prompt: 👎

Name [default]:

Ideally, this would also show the show_default:

Name [show_default]:

Environment:

• Python version: 3.11.6
• Click version: 8.1.6