Add Serifed Double-struck Capital Letters

[briansmith]

I understand that sans-serif "blackboard bold" makes sense because people don't generally draw serifs when writing on a blackboard. However, IMO serif double-struck capital letters don't blend in well with serif body text. It would be great to have serif versions available, either as the default (my preference) or as a variant.

[xsrvmy]

Adding to this, another useful style would be the outline style that AMS fonts use.

[tiroj]

@davidmjones Any thoughts on this? These, if considered for addition, would presumably need to be implemented as variants of the double-struck characters?