Support spread in attributes

[mlrawlings]

New Feature

Description

Support the spread operator in tag attributes.

<tag ...attrs/>

Why

We currently support two ways of passing dynamic attributes to a tag:

<custom-tag(input.attrs) data-foo=input.foo/>
<custom-tag ${input.attrs} data-foo=input.foo/>

Attributes should be the only way in which we pass data into tags (except for a few exceptions like if and for). The spread attribute is soon to be a JS standard and has a major benefit over the current solutions: order matters.

Using the parens as a tag argument (<custom-tag(input.attrs)/>) prevents you from ordering and even <custom-tag ${input.attrs}/> which could support ordering, doesn't.

Ordering gives you the ability to assign precedence, to create both default values and override values.

<custom-tag foo="default" ...input bar="override"/>

As a side note, the parens as an argument don't work for regular dom tags.

// no matter what `input.attrs` is, all you get is `<div data-foo>`
<div(input.attrs) data-foo=input.foo/>