add debug layer

[sukeyisme]

add realgud debuger for c++. related: #5910

[dvzubarev]

Thank you, I've tested this PR and it worked quite well for c++. I would suggest to add realgud-short-key-mode to e.g. ,ds. I found myself in need to enable this mode often (when I set breakpoints) or disable it for a while to fully use evil-mode in a buffer.
Are you able to activate a backtrace buffer: M-x realgud:window-bt or F in a source file. It hangs for me, I can only recover by spamming C-g.

[sukeyisme]

@dvzubarev May be a bug. realgud/realgud#155

[syl20bnr]

Thank you ! 👍
Since the layer is only configured for C and C++ for now I just moved the realgud configuration to the c-c++ layer. See commit 7d215cb0
Cherry-picked into develop branch, you can safely delete your branch.