Support unicode SI prefix µ

[mia-trilo]

Currently, it appears the short SI prefix for $10^{-6}$ is ASCII letter u instead of the "proper" prefix µ, as documented on Wikipedia. On a macbook, you can type this by pressing option-M. It would be lovely if there is support for this prefix, or if there is custom prefix enabled.

Currently, I have a workaround of defining custom units. E.g. for liter, I have the following snippet

const isAlphaOriginal = mathjs.Unit.isValidAlpha;
mathjs.Unit.isValidAlpha = function (c) {
    return (
        isAlphaOriginal(c) ||
        c.charCodeAt(0) === 181 ||
        c.charCodeAt(0) === 956
    );
};
mathjs.createUnit(String.fromCharCode(181) + "L", mathjs.unit(1,"uL"));
mathjs.createUnit(String.fromCharCode(956) + "L", mathjs.unit(1,"uL"));

However, doing so means the automatic prefix conversion doesn't work as intended. For example, 10000000µL is formatted as 1e+7 µL instead of 10 L

[gwhitney]

Added this very reasonable suggestion to #3365. (At the moment, mathjs has reasonable unicode capacity for extensions and your own-code but very little in the way of unicode-based functionality built into it out of the box.