[lldb-vsocde] Add a 'continued' event for programmatic continue events.

When the process is contiuned using an lldb command expression the thread state in VS Code is never informed and will be out of sync with the current state of the process. The new event will fire whenever the process is continued and keeps the debugger in sync with the dap client.

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D154989