Add capital B for \mathbb in cdlatex-math-modify-alist-default

[t-c-acc]

The \mathbb style is verry common in mathematics, for example $\mathbb{R}$, $\mathbb{Z}$, $\mathbb{F}_p$, etc. In the past it was recommended not to use \mathbb in print, and only use it on blackboards, but this has changed and it is now very common in books and papers.

It seems to me that it should be included by default as an option for cdlatex-math-modify. (\mathsf is included, which is certainly less common)

The capital letter B seems like a sensible choice, since it is not already taken, and since \mathbb stands for "Blackboard Bold".