add scope attribute to th tag

[JacobHearst]

Exactly what it says on the tin, adds the scope attribute for th elements.

[sliemeobn]

thanks @JacobHearst for the contribution!

since scope is only used on th, can I ask you to add the extension on the tag type directly?

the dance through a "trait" protocol is simply unnecessary and other attributes also follow this pattern for "single tag usage".

[sliemeobn]

🙏