Get signs from all groups

[aktau]

Currently vim-signjump uses the following to get a list of all placed signs in a buffer:

split(execute('sign place buffer='.a:buffer, 'silent'), '\n'), 

However, this doesn't work when a plugin has placed signs in their own group (an example is vim-lsp).

Changing this to:

split(execute('sign place group=* buffer='.a:buffer, 'silent'), '\n'), 

Fixes it. I haven't been able to observe an ill effect in vim-signjump.

[ZeroKnight]

group seems to be part of the new API, so this can be applied after handling #14.

[aktau]

It seems like group=* functionality (and the new API) was included in patch 8.1.0658. There were some hotfixes afterwards, but I think none that affected the fetching of group functionality.

These patches introduce both the new API and extend the old Ex command API, so indeed it would be wiser to have a cutoff point:

1. Old API without groups for Vim versions pre-8.1.0658, or Neovim versions pre-exists('*sign_define')
2. New API with groups for newer versions.

So yes, this can be done after or during #14.