TeX-like mathbf (and similar commands) only works on identifiers

[Omikhleia]

Issue found by @pekkavaa

Observed

The following math formula...

$`\mathbf{x_i}`

.... crashes with an error.

Workaround

One can use \mathbf{x}_\mathbf{i} i.e. mathxxx variant-changing commands only on identifiers.

Analysis

It is due to our interpretation of \mathbf (and other similar commands) as very simple macro definition, only working on final identifiers (i.e. as MathML <mi>), not around full expressions:

\def{mathbf}{\mi[mathvariant=bold]{#1}} % ---> so %1 here has to be a string/symbol identifier

For the record, MathJax supports it, but by propagating the property recursively down to the inner MathML identifiers and operators (if I understand well how it does it):

This would require a higher-level hard-coded logic in the LPEG parser, probably, to get it right (e.g. perhaps not wrapping anything, but handling a stack for applying the math variant on inner <mi>'s and mo's (if they have none explicit) -- that's just a wild guess...)

Extracted from #2291