Include CSS content property in accessibleText

In axe-core 4.5 we deprecated support for IE11. Part of that means we should no longer be failing elements that have their accessible name created from the CSS content property. To do so, axe-core's accessibleText calculation will need to include CSS content. Something like the following should pass the `button-name` rule:

```html
button:before { Content: "Hello world" }

```