feat(math): Minimal support for accents in MathML and TeX-like commands

[Omikhleia]

Closes #2150

Description

1. So let's first be positive, regarding what this PR brings:

• MathML accent/accentunder on mover/munder/munderover elements
• Many top and bottom accents "quite well placed" (no longer floating off the formula): accents, arrows, etc.
• Proper syntax support for these in TeX-like math syntax
• TeX-like "variant" limits (varlimsup, varliminf, varprojlim, varinjlim)
• Horizontal braces, brackets and parentheses, with appropriate TeX math support (over/underbrace, over/underbracket, over/underparen)

That's not too bad, from a user point of view.
IMHO, we wouldn't fare long without accents, and this is exactly what this PR solves on the surface, leaving room for further improvements (see below).

2. The implementation is slightly more controversial:

• Use of combining vs. non-combining Unicode code points (ideally, we likely need both, with additional conversion tables).
• Use of horizontal re-scaling when glyph construction is not possible (partly depends on the previous point).
• Incomplete implementation of MathML Core's layout recommendations. (Well, this was already the case, but it needs to be mentioned: This is not fully closing the gap)
• Some internal flags introduced as MathML specifications still seems in the flux regarding these topics...

This being said, we are closer to the goal, and there's a limit on how much one can do with unfinished and evolving standards. I tend to believe something towards the goal is better than nothing at all, and that the benefits for being able to showcase the above features is greater than waiting for a better implementation that might never come?

What do you think?