Plugin txt description

[Pshy0]

This implements the new about property in plugin.txt.

[TheGiraffe3]

This could be merged now as the plugin.txt file has been in many releases.

[Pshy0]

This file has indeed been implemented for many releases now.

I not only delayed merging this because the old syntax is still supported while the new one was giving errors in older versions, but also because this feature was not allowing multiline abouts at the time (a regression). I do not know if this have since been fixed.

[TheGiraffe3]

It has.

[Pshy0]

After testing, I confirm the multiline about feature in plugin.txt have still not been fixed.
This PR will be further delayed.

[Pshy0]

endless-sky/endless-sky#10970

[Pshy0]

After investigating, multiline abouts are working like this:

about `Ruin The Fun (CHEAT PLUGIN)`
about `  A development / cheat plugin to explore and try out all content.`
about `  Access all ships and outfits, change your captain's name, choose the color swizzle of your ships, edit your relation with other governments, set your combat rank, hire a powerful escort, gift yourself pre-outfitted ships, give yourself money, and much more.`

However, documentation says that it's not supported, so I'll wait that either documentation gets fixed or this syntax gets implemented in a stable release.