Expose helper methods on StyledStr for easy use by consumers

[pdaoust]

Please complete the following tasks

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

Clap Version

master

Describe your use case

Sometimes there's a desire to create extra help text (e.g., in before_help) that's similar in structure to the autogenerated help text but written by hand. By 'similar in structure' I mean that, for example, headers/literals match the colours of autogenerated headers/literals and multi-line text is wrapped and indented properly.

There's already PR #4765 to allow StyledStr to accept user-styled text and output the proper escape symbols for the execution context. The advantage of using the already existing helpers is that it's durable to changes -- if the maintainers of clap decide to change headers from yellow/uppercase to blue/title-case, the manually written text will change too.

I'd be happy to submit a PR for this, but thought I'd open it as an issue in case discussion should happen first.

Describe the solution you'd like

StyledStr::header(), ::literal(), ::warning(), ::trim(), ::indent(), and so forth are exposed as public members of the impl and are made available to projects that depend on the clap crate.

Alternatives, if applicable

No response

Additional Context

No response

[epage]

The existing API won't work when we move forward with #3234, so I'm going to close this as Not Planned,. If there is a reason for us to re-evaluate this, let us know!