Use data-targets for @targets

[muan]

Given

@targets filters: HTMLElement[]
@target allFilter: HTMLElement

Currently this is the markup

<div data-target="foo-controller.filters foo-controller.allFilter"></div>

I propose that we add data-targets, like so:

<div data-targets="foo-controller.filters" data-target="foo-controller.allFilter"></div>

In this case we see from the markup that this is supposed to be the only one allFilter, but there should be multiple filters.

I think data-targets will

1. Match with the decorator thus make it clearer that to find a collection you need @targets in TS.
2. Provide better context when looking at the markup. When refactoring unfamiliar, JS we often have to check "is this thing rendered more than one time?"