[css-cascade-5] Add a R/W `CSSStyleSheet.layer` attribute for the layer name

(Edit: fixed some details on settings)

I'm proposing extending the CSSStyleSheet interface with a new attribute, layer.

On getting, it should return the layer name if the sheet is entirely in some cascade layer. In particular:

• If this is an @import-ed stylesheet, return the layerName attribute value of the CSSImportRule
• If this stylesheet is added via a <link> element (depending on https://github.com/whatwg/html/issues/7540), then return the name of that layer
• If this is a constructed stylesheet with a layer name previous set by JS (see below), return that layer name
• Otherwise, return null

On setting:

• If this is a constructed stylesheet and the new value is a non-null string, then the attribute value can be modified, and the stylesheet is treated as entirely in the given layer
• If this is a constructed stylesheet and the new value is null, then the attribute value can be modified, and the stylesheet is treated as unlayered
• Otherwise, throws a NotAllowedError DOMException

The primary use case is to allow adding layered constructed stylesheets, especially if the stylesheet is a 3rd party CSS module, and we want to use that to set up a web component (which we currently don't have a good way to make it layered):

import sheet from 3p-foo.css;

sheet.layer = 'new-layer';
shadowRoot.adoptedStylesheets = [sheet];

Start by locating the CSSStyleSheet and CSSImportRule definitions in the CSSWG draft, then review the dependency on WHATWG HTML issue #7540 for behavior. Done means the getter, setter, layer-treatment, and NotAllowedError requirements are resolved in the relevant specification text with working-group agreement.