Add terminateCommands to lldb-vscode protocol

Summary: Adding this in line with "stopCommands" and "exitCommands" so that we can run commands at the end of the debugging session.

Reviewers: clayborg, wallace, labath

Reviewed By: clayborg, labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D79726