Add :host rule to @theme layer (#15975)

Resolves #15799
Resolves #14478
Part-of #15005

Adds a `:host` selector for the `@theme` layer. This is necessary for
the `@theme` layer to work correctly in shadow DOM.

Also updates the snapshots for the tests that were affected by this
change (in a separate commit).

## Test plan

Tested via the Vite playground:

<img width="1121" alt="Screenshot 2025-01-29 at 15 06 49"
src="https://github.com/user-attachments/assets/a7908135-5ff8-472f-a053-d2c6d5c81e1b"
/>

Additionally made sure that `@property` defaults also work across
Firefox, Chrome, and Safari (the `@property` definition from the root is
pulled in) and added a UI spec.

---------

Co-authored-by: Philipp Spiess <hello@philippspiess.com>

integrations/cli/index.test.ts

@@ -370,7 +370,7 @@ describe.each([
 
       await fs.expectFileToContain('project-a/dist/out.css', [
         css`
-          :root {
+          :root, :host {
           }
         `,
       ])