`\mathbf` produces bolditalic instead of bold (LaTeX vs ISO standard)

Currently, `L"\mathbf{x}` gets parsed to `TeXChar 'x' [index 89 in NewComputerModern - 10 Bold Italic]`.
This deviates from the standard LaTeX behavior, where we would expect an upright bold character.

In fact, the LaTeX package `unicode-math` offers options (`math-style` and `bold-style`) to conform to different notational standards. `MathTeXEngine` currently mostly adheres to the "ISO" style (except for Greek letters, as per https://github.com/Kolaru/MathTeXEngine.jl/issues/87).
I guess, this is due to `_default_font_mapping[:char]=:italic`?
I think, in the long run it would be great to also enable the user to choose either norm (like unicode-math does) or be consistent with the LaTeX style.

I tried
```julia
__default_font_modifiers = deepcopy(_default_font_modifiers)
__default_font_modifiers[:bf] = Dict(
:italic => :bold,
:regular => :bold
)
MYFONT = FontFamily(
_new_computer_modern_fonts,
_default_font_mapping,
__default_font_modifiers,
_symbol_to_new_computer_modern,
13,
0.0375
)
```
as a quick hack, which indead works as expected: `generate_tex_elements(L"\mathbf{x}", MYFONT)[1][1]`
gives `TeXChar 'x' [index 89 in NewComputerModern - 10 Bold]`.
Unfortunately, it seems that I cannot pass my `MYFONT` down to be used with labels in Makie.