Warn when assigning custom attribute via interpolation

[MeirionHughes]

<div something="${foo}"></div>

should raise an issue (not supported)

[atsu85]

I guess if tag has also as-element attribute containing valid custom element tag name, then it would be ok. For example:
<div something="${foo}" as-element="my-custom-element"></div>

[MeirionHughes]

Good point. Also I guess would need to rule out all custom-attribute handlers too.