Compatibility: Add a definition of \boldsymbol

[SFr682k]

Description

Although the \boldsymbol command is deprecated, it is still used by some math packages.
Therefore, it would be nice to have a \boldsymbol command in unicode-math, which produces bold italic symbols independent from the current value of bold-style.

Check/indicate

• [X] Relevant for XeTeX
• [X] Relevant for LuaTeX
• [X] Issue tracker has been searched for similar issues?
• [ ] Links to <tex.stackexchange.com> discussion if appropriate

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{amsmath}
\usepackage{unicode-math}
\setmathfont{latinmodern-math.otf}
\begin{document}
    $\boldsymbol{x y z}$
\end{document}

Further details