Improve RichFormatter's multi-line error formatting

[mahor1221]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.5.7

Describe your use case

I return multi-line errors in some parsers, which causes minor formatting issues when using RichFormatter. For example:

#[derive(Parser)]
pub struct Cli {
    ...
    #[arg(long, short, value_parser = TransitionScheme::from_str)]
    #[arg(value_name = "TIME-TIME | DEGREE:DEGREE")]
    pub scheme: Option<TransitionScheme>,
    ...
}

Current format:

$: cargo run -- --scheme asdf:ghjk
error: invalid value 'asdf:ghjk' for '--scheme <TIME-TIME | DEGREE:DEGREE>': as time ranges:
- invalid format
as elevation range:
- invalid float literal (asdf)
- invalid float literal (ghjk)

For more information, try '--help'.

Desired format:

$: cargo run -- --scheme asdf:ghjk
error: invalid value 'asdf:asdf' for '--scheme <TIME-TIME | DEGREE:DEGREE>': # a new line is added here
as time ranges:
- invalid format
as elevation range:
- invalid float literal (asdf)
- invalid float literal (ghjk)

For more information, try '--help'.

Describe the solution you'd like

Please note that I'm not very familiar with the clap's code base. Should this feature even be added or not?

A possible solution (but most likely a breaking change) would be to check if the formatted source error is multi-line, and if so, add a newline character to ensure proper formatting.

Another possible solution would be to make RichFormatter configurable.

Alternatives, if applicable

I've created a patch for my own use case here. It was very hard to make :)
https://github.com/clap-rs/clap/compare/v4.5.7...mahor1221:clap:patch?expand=1

Additional Context

Here is the location of the issue:

clap/clap_builder/src/error/format.rs

Line 370 in 6c6839a

let _ = write!(styled, ": {source}");

[epage]

This is a tough situation because it happens to be a problem in your case due to the lack of alignment but in other cases it can be neutral or better the existing way.