[lldb-vscode] Adding support for displaying backtraces.

On Apple platforms when debugging with libBacktraceRecording.dylib backtraces are stored as part of the thread stack. This change includes support for displaying the back traces when they are present in the stack trace.

To use this on macOS a binary needs to be run with the following environment variables configured:

DYLD_LIBRARY_PATH=/usr/lib/system/introspection
DYLD_INSERT_LIBRARIES=/Applications/Xcode.app/Contents/Developer/usr/lib/libBacktraceRecording.dylib
{F28473587}

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D156465