Add outline-center, outline-inner and outline-outer utilities

[deebov]

Problem: Many design tools like Figma have an option to configure element's stroke (border) position such as "inner", "outer" and "center". Currently, it's not possible to set border's position in CSS.

Solution: This can be achieved with outline and outline-offset.

For example:

.outline-4 {
    --outline-width: 4px;
    outline-width: var(--outline-width);
}

.outilne-outer {
    outline-offset: 0px;
}

.outilne-inner {
    outline-offset: calc(-1 * var(--outline-width)); // -4px
}

.outilne-center {
    outline-offset: calc(-0.5 * var(--outline-width)); // -2px
}

I can see that this might be implemented using a plugin but would you consider adding this feature in the future? Or at least using a variable for outline-width so it's easier for a plugin to implement this feature.