Since Neovim commit c4afb9788c4f139eb2e3b7aa4d6a6a20b67ba156, the sign
API uses extmarks internally. Virtual text is already rendered using
extmarks. ALE uses the same group name for both signs and virtual text
and as a result, both are placed in the same extmark group. Since ALE
deletes all extmarks in the virtual text group after all signs have been
placed, no signs are ever shown. This commit fixes this by renaming the
sign group from `ale` to `ale_signs`.
* Use sign-group only on supported vim versions.
The sign-group feature is only available in nvim 0.4.0 and vim 8.1.614.
* Add priority to ALE signs.
This allows users to set a priority to ALE signs to take precedence over
other plugin signs.
