support variable property names

[benmusson]

Fixes #97. Allows the use of variable in declaration property names.

Example:

$name: custom-property;
$value: black;

.class {
    --$(name): $value;
    color: var(--$(name))
}

results in

.class {
    --custom-property: black;
    color: var(--custom-property)
}

[jonathantneal]

Wow, this looks great. Thanks, @benmusson! Let’s cut a release soon. I wish my PRs automatically ran the tests. Maybe I can ask a robot to write the actions yml file for that. 😂

[benmusson]

I wish my PRs automatically ran the tests

https://github.com/csstools/postcss-advanced-variables/pull/101