refactor(styles): make styles example use a const

[gibfahn]

I was trying to set up colours for the derive macro, and copying this didn't immediately work, but using .bold() as per #3234 (comment) did work, so figured it would be worth updating the defaults so things are a bit easier for the next person.

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Commits (oldest to newest)

6f215ee refactor(styles): make styles example use a const

This makes it easier to copy this example for use in the derive API,
like so:

const STYLES: Styles = Styles::styled()
    .header(AnsiColor::Green.on_default().bold())
    .usage(AnsiColor::Green.on_default().bold())
    .literal(AnsiColor::Blue.on_default().bold())
    .placeholder(AnsiColor::Cyan.on_default());

#[derive(Parser)]
#[clap(styles = STYLES)]
struct Cmd {
  ...
}

If you use the | method then it's not a constant function.

[epage]

Looks like this needs to bump the version of anstyle to pass CI

[epage]

Thanks!