[RFC] Custom element bindables take priority over custom attribute

[bigopon]

🔦 Context

Suppose we have a global custom attribute size, and a custom element <square> with a bindable size property, and the following template:

<square size.bind="width">

Currently, in both v1 and v2, size.bind is understood as custom attribute size and thus the property size on the custom element <square> will be ignored. This will also lead to a different behavior when used in spreading ...$attrs.

This order of custom attribute taking over custom element bindable is not intuitive as it allows any arbitrary global custom attribute to override local element without anyway to overcome it, especially when consuming a 3rd party plugin. It also doesn't reflect common expectation of local first behavior.

In v2, we should change it so that custom element bindable will be of higher priority than custom attribute.

❔ Migration

Migration shouldn't be a big issue since it's unlikely that app will keep the conflict around.

cc @Sayan751 @fkleuver