[lldb-vscode] Fix focus thread when previous thread exits

The thread that Visual Studio Code displays on a stop is called the focus thread. When the previous focus thread exits and we stop in a new thread, lldb-vscode does not tell vscode to set the new thread as the focus thread, so it selects the first thread in the thread list.

This patch changes lldb-vscode to tell vscode that the new thread is the focus thread. It also includes a test that verifies the DAP stop message for this case contains the correct values.

Reviewed By: clayborg, wallace

Differential Revision: https://reviews.llvm.org/D109633