Add MathML Core

[Schweinepriester]

• https://www.w3.org/TR/mathml-core/
• https://www.igalia.com/2021/10/19/Announcing-the-MathML-Core-Support-Collective.html

For distinction to MathML:

• https://www.w3.org/TR/mathml-core/#introduction
• https://www.igalia.com/chats/mathml-core-collective#:~:text=specs%20back%20in%20those%20days%20were%20a%20lot%20more%20aspirational. ff.

Maybe there is a discussion to be had if this should be it's own entry, but I would argue for yes, because way more standardized.

[Schweinepriester]

Shipping in Chrome 106: https://chromestatus.com/feature/5240822173794304