if (type !== "textord" &&
        lookupSymbol(value, "Math-BoldItalic", mode).metrics) {
        return {
            fontName: "Math-BoldItalic",
            fontClass: "boldsymbol",
        };
    }
Determines which of the two font names (Main-Bold and Math-BoldItalic) and
corresponding style tags (mathbf or boldsymbol) to use for font "boldsymbol",
depending on the symbol.  Use this function instead of fontMap for font
"boldsymbol".