Add support for `\symbf` and related commands from the unicode-math package

> Before requesting a feature, please search for [existing issues](https://github.com/KaTeX/KaTeX/issues).

Searching for `symbf` resulted in zero issues.

Searching for `unicode-math` revealed some potentially related issues, but they are fundamentally different:

- #3212
- #564
- #3503
- #260

**Is your feature request related to a problem? Please describe.**

KaTeX does not support `\symbf` and related commands from [unicode-math](https://github.com/latex3/unicode-math). This is inconvenient when writing equations that should work with both KaTeX and `unicode-math`

**Describe the solution you'd like:**

Most `unicode-math` commands can be implemented by assigning them the corresponding existing commands, e.g. `\symbf` can be treated as synonymous to `\mathbf`.

**Link to or name of a (La)TeX package that provides the same feature:**

https://github.com/latex3/unicode-math

**Describe alternatives you've considered:**

- `\pmb`, but this far from clean.
- Directly writing in unicode (is in #260) is impractical.

Start by reviewing KaTeX's existing command definitions and the unicode-math package's related-command list. Confirm which \symbf and related commands should map to existing equivalents, then verify that representative equations render correctly in KaTeX.

Written by the indexing model from the issue text.