Add $derive:property directive

[hawkticehurst]

Feature description

Potentially add a $derive:property directive that will derive new state based on existing state(s), tied to an element property.

Syntax

$derive:property="callback(...statenames)"

Usage

Like the $derive directive, $derive:property will be a way of creating new state based on other state in a Stellar component, but the new state will be tied to an element property instead of element text content.

Notes

I'm still not sure if this is needed or not.

Goal: Can I find use cases/scenarios where we need a reactive property that is dependent on other states?