Add json5 support

[Sean Escriva]

Hi there,

Has json5 support been looked at for this major mode? I'm starting to see these files used as configuration, notably of course with VSCode but also tools like Renovate and it would be nice to have major mode support for it.

I found some early discussion about implementing support https://gitter.im/emacs-lsp/lsp-mode?at=5f9accbdf2fd4f60fc3f6bc7 but haven't found anywhere that was followed through on other than in this repo: https://github.com/nbfalcon/json5.el

Is anyone aware of work done in this area? Would it be best implemented in this mode or as a brand new mode?