feat(math): Support MathML mpadded and refactor TeX-like phantoms

Nothing really complex with mpadded.. Wait some ambiguity if dimensions
are in a font-relative unit (see in-code comment), but let's move
forward anyway -- and on the same occasion, remove the non-standard
approach previously used for (La)TeX's phantom, hphantom and vphantom.

packages/math/texlike.lua

@@ -661,8 +661,9 @@ compileToMathML(
 
   % Phantom commands from TeX/LaTeX
   \def{phantom}{\mphantom{#1}}
-  \def{hphantom}{\mphantom[special=h]{#1}}
-  \def{vphantom}{\mphantom[special=v]{#1}}
+  \def{hphantom}{\mpadded[height=0, depth=0]{\mphantom{#1}}}
+  \def{vphantom}{\mpadded[width=0]{\mphantom{#1}}}
+  %\mphantom[special=v]{#1}}}
 ]==],
    })
 )

packages/math/typesetter.lua

@@ -177,6 +177,10 @@ function ConvertMathML (_, content)
       -- It's an mrow, but with some style attributes that we ignore.
       SU.warn("MathML mstyle is not fully supported yet")
       return b.stackbox("H", convertChildren(content))
+   elseif content.command == "mpadded" then
+      -- MathML Core 3.3.6.1: The <mpadded> element generates an anonymous <mrow> box
+      -- called the "impadded inner box"
+      return b.padded(content.options, b.stackbox("H", convertChildren(content)))
    else
       SU.error("Unknown math command " .. content.command)
    end