Feature: support styled spans within error messages

[o6uoq]

Currently error text is rendered as a single styled block (title-cased first word + ErrorText style).

It would be useful to allow inline styling within error messages e.g. colorising flags (--thing) or env vars (ENV_VAR) differently from the surrounding text.

Today the only way to achieve this is embedding raw ANSI codes in the error string or bypassing fang's error rendering entirely, both of which are hacky.

A simple API like fang.Highlight("--thing") that returns a string passable into errors.New / fmt.Errorf would cover most cases.