contrib/vscode/: debugging with VS Code and gdb

contrib/vscode/: debugging with VS Code and gdb

The externalConsole=true setting is broken for many users (launching the
debugger with such setting results in VS Code waiting forever without
actually starting the debugger). Also, this setting is a matter of user
preference, and is arguably better set in a "launch" section in the
user-wide settings.json than hardcoded in our script. Remove the line to
use VS Code's default, or the user's setting.

Add useful links in contrib/vscode/README.md to help the user to
configure VS Code and how to use the debugging feature.

Helped-by: Matthieu Moy <Matthieu.Moy@univ-lyon1.fr>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Co-authored-by: BRESSAT Jonathan <git.jonathan.bressat@gmail.com>
Signed-off-by: COGONI Guillaume <cogoni.guillaume@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>