Creating a startDebugging reverse DAP request handler in lldb-vscode.

Adds support for a reverse DAP request to startDebugging. The new request can be used to launch child processes from lldb scripts, for example it would be start forward to configure a debug configuration for a server and a client allowing you to launch both processes with a single debug configuraiton.

Reviewed By: wallace, ivanhernandez13

Differential Revision: https://reviews.llvm.org/D153447