highlight linenr without enabling signs?

This request is similar to https://github.com/dense-analysis/ale/issues/741, except that I'd like to use line number highlighting (`g:ale_sign_highlight_linenrs`), but not have Ale touch the signcolumn(s).

The reason being:

1. highlighting an entire line is too obtrusive
2. I'd like to keep the sign columns reserved for other plugins, and use line number coloring for Ale specifically.

I can create a PR when I find some time, but wanted to make sure this is something you're willing to entertain.

Start by reviewing the related issue 741 and the existing g:ale_sign_highlight_linenrs option. Trace how ALE currently applies line-number highlighting and sign-column changes; done means line-number highlighting can be enabled without ALE modifying the sign columns.

Written by the indexing model from the issue text.