Add a "tab" strip for open dart source code windows.

[julian-yang]

It'd be great if we can have a "tab" strip for open source code tabs, similar to what chrome inspector has today: https://snipboard.io/tNep7n.jpg

Or even if there was a dropdown menu / 3dot menu to show previously opened tabs. Right now we're restricted to either re-searching for the file via ctrl-p, or clicking the left/right arrows.

[kenzieschmoll]

Here is some prior art for a "tabbed" view that was built for the Performance page. It has since been removed, but some of the code from this PR may be reusable for a feature like this in the debugger. https://github.com/flutter/devtools/pull/3509