[m-rev.] for post-commit review; start using color for the subjects of diagnostics

[Julien Fischer]

On Sun, 5 May 2024, Zoltan Somogyi wrote:

> Color the subjects of error messages.
> 
> compiler/error_spec.m:
>     Add a new color name, color_subject, which is intended to be used
>     as the color for the subject of an error message. The subject is
>     usually a variable or a term.
> 
> compiler/options.m:
> doc/user_guide.texi:
>     Add an option to control the actual color for this color name.
> 
> compiler/globals.m:
>     Include the new color in the color specification database.
> 
> compiler/write_error_spec.m:
>     Handle the conversion of the new color name to the selected
>     corresponding actual color.
> 
> compiler/typecheck_errors.m:
>     Color the subjects of (most of) the error messages generated by
>     this module with the new color.

I've just stepped through a bunch of the new outputs with the latest ROTD.
I think coloring the subject is an improvement.  The diff is fine.

Julien.