servoshell: add support for tabbed browsing

[delan]

This patch adds support for tabbed browsing in servoshell’s minibrowser, like #30785 but rebased onto #31417 and simplified to avoid current multiview and egui limitations.

For now, only one webview will be focused and shown at a time. To close a tab, click the ❌︎ in the tab bar, or middle-click the tab itself. When closing a tab that is currently focused, we switch to the next most recently focused tab.

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because ___