[vscode] set default values for terminateDebuggee for the disconnect request

Summary:
Recently I've noticed that VSCode sometimes doesn't send the terminateDebuggee flag within the disconnectRequest,
even though lldb-vscode sets the terminateDebuggee capability correctly.
This has been causing that inferiors don't die after the debug session ends, and many users have reported issues because of this.

An easy way to mitigate this is to set better default values for the terminateDebuggee field in the disconnect request.
I'm assuming that for a launch request, the default will be true, and for attach it'll be false.

Reviewers: clayborg, labath, aadsm

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D81200