Ability to set suggestion for empty prompt or an argument with suggestion to prompt()

[anki-code]

The feature request came from xonsh shell - xonsh/xonsh#6044

When we call prompt() method we can't set default buffer suggestion. But it would be very helpful for AI suggesters or extracting next command from history, etc.

The minimal example of use case:

prompt> echo 1
prompt> echo 2
prompt> echo 3
prompt> echo 1
prompt> <cursor>echo 2  # Here `echo 2` is the suggestion (grey text).

For community

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[anki-code]

The workaround is to use handler like in xonsh/6048:

def handler_before_render(app):
    if not app.current_buffer.text and (suggestion := XSH.env.get('XONSH_PROMPT_NEXT_CMD_SUGGESTION')):
        app.current_buffer.suggestion = Suggestion(suggestion)
        XSH.env["XONSH_PROMPT_NEXT_CMD_SUGGESTION"] = ""
self.prompter.app.before_render.add_handler(handler_before_render)