Behaviors should accept a Promise sheet type

[nicholasrice]

The behavior returned from matchMediaStylesheetBehaviorFactory currently accepts an ElementStyles object.

If we allow this to additionally accept a Promise<ElementStyles> type then this behavior could be used with dynamic import syntax: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import. In practice, this means that forced-color stylesheets could be dynamically loaded and not shipped in the primary bundles:

Previous

const styles = css` ....`
    .withBehaviors(forcedColorsStylesheetBehavior(css`/* high contrast styles */`))

Proposed additional behavior

const styles = css` ....`
    .withBehaviors(forcedColorsStylesheetBehavior(import("./high-contrast-stylesheet.js"))

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.