Extend mpadded attribute syntax to be closer to MathML3

[fred-wang]

In #81, we agreed that mpadded attributes in MathML Core should only use pseudo-unit with the same vertical/horizontal direction.

Can we simplify it any further? Remove/Deprecate it?

Some thoughts:

• MathML discourage authors from using mpadded for tweaking or conveying meaning: https://mathml-refresh.github.io/mathml/chapter3.html#presm.warnfinetuning
• In general, since MathML3 did not clearly specify rendering, implementation are inconsistent and usage is probably unreliable.
• CSS is more appropriate for this kind of space tuning. I imagine if we better define MathML interactions with out-of-flow position Handling of out-of-flow elements mathml-core#74, margin/border/padding Ensure good integration of MathML with the accessibility tree #9, vertical-align CSS interoperability with vertical-align mathml-core#273, aspect-ratio etc authors could use these alternatives.
• mpadded is probably used a lot, though. We would need to get better statistics and analysis of current use cases.

[fred-wang]

These are the possible non-length values implemented by @distler in itex2MML and copied into TeXZilla:
lspace="-100%width" (\mathllap)
lspace="-50%width" (\mathclap)
height="+[LENGTH][UNIT]" (\mathraisebox)
depth="+[LENGTH][UNIT]" (\mathraisebox)
depth="depth" (\mathraisebox)

depth="depth" can basically be replaced with not using (or ignoring) the attribute. I'm not exactly sure about how the other commands work.

[fred-wang]

From https://mathml-refresh.github.io/mathml/chapter3.html#presm.mpadded

For any unsigned number X, if Y = X / 100 then

X%pseudo-unit is equivalent to Ypseudo-unit
+X%pseudo-unit is equivalent to +Ypseudo-unit
-X%pseudo-unit => -Ypseudo-unit

So it seems that this syntax is not useful.

[fred-wang]

I've rewritten the description of mpadded in MathML core, so that it no longer refers to MatHML3
https://mathml-refresh.github.io/mathml-core/#adjust-space-around-content-mpadded

Unless I'm mistaken, the current syntax and semantic should more or less match what is in the full spec:
https://mathml-refresh.github.io/mathml/#presm_mpaddedatt
(with namedspace removed and numbers defined from CSS)