Make accessibility an option

In an effort to make Gridicons an effective icon system, I think it would great to include a way to make the icon accessible as an option.

Take a look at this article:

https://css-tricks.com/can-make-icon-system-accessible/

Icons can be either **standalone** or **decorative**.

Standalone needs to be accessible and this is the standard pattern for that:

```html

```

Decorative just needs `aria-hidden="true`

```html

Add to Cart

```

It would be great if we could somehow bake this into the icon system, instead of pushing that off to the user. You could imagine something like this working:

``

I'm not sure how we would go about this.

The issue names no files, tests, or entry points; start by locating the Gridicons rendering entry point and reviewing the linked accessibility article. Define how standalone and decorative icons are selected and what accessible SVG output should be verified before implementation.

Written by the indexing model from the issue text.