Add fence and separator attributes to <mo> MathML element

[pepelsbey]

Summary

Adds missing fence and separator attributes to <mo> MathML element

Test results and supporting details

• They are available in the MathML Core spec
• They are already documented on MDN

Note

Only when sending the PR did I realize that for each one of them:

There is no visual effect for this attribute

Which means there’s no way to tell if they’re supported. Unless browsers might convey them somehow to a screen reader. Now or in the future. At the same time, it’s a way to complete BCD and document the standard status of the feature.

Given all that, I used the lowest MathML implementation version of all “supporting” browsers.

[caugner]

The spec states:

This specification does not define any observable behavior that is specific to the fence and separator attributes.

So I'm wondering if it makes sense to document them in BCD?