Property autocomplete + syntax highlighting does not trigger inside custom directive blocks

If you add a new directive to CSS custom data, autocomplete + syntax highlighting does not work within blocks defined by the directive. Mixins are one example where this would be useful:

@define-mixin sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    white-space: nowrap;
    border-width: 0;
}

While this behavior may not be desirable for all directives, it'd be nice to have this as an option for some of them.

Start with docs/customData.md and the custom directive behavior described in the issue. Trace how directive blocks affect property autocomplete and syntax highlighting, then identify the existing coverage for those features; done means selected custom directives can opt into both behaviors without changing directives where that behavior is undesirable.