Use transparent background for gutter line number in GUI mode (#204)

The `LineNr` and `CursorLineNr` highlight groups now have a transparent
background in GUI mode.

Before it was set to `nord0_gui` which worked fine in most cases.
However, some plugins use these highlight groups to render their content
in a popup window which can potentially have a different background
color. This caused some issues e.g. for the fuzzy search plugin
LeaderF [1].

The compatibility with the `g:nord_cursor_line_number_background`
theme configuration has been verified to work as expected in both modes
when it is set to `0` or `1`.

This change is not related to the terminal mode or when using
`set notermguicolors` since `ctermbg` for `LineNr` and `CursorLineNr`
is set to `NONE` by default.

[1]: https://github.com/Yggdroot/LeaderF

colors/nord.vim

@@ -170,7 +170,7 @@ call s:hi("Cursor", s:nord0_gui, s:nord4_gui, "", "NONE", "", "")
 call s:hi("CursorLine", "", s:nord1_gui, "NONE", s:nord1_term, "NONE", "")
 call s:hi("Error", s:nord4_gui, s:nord11_gui, "", s:nord11_term, "", "")
 call s:hi("iCursor", s:nord0_gui, s:nord4_gui, "", "NONE", "", "")
-call s:hi("LineNr", s:nord3_gui, s:nord0_gui, s:nord3_term, "NONE", "", "")
+call s:hi("LineNr", s:nord3_gui, "NONE", s:nord3_term, "NONE", "", "")
 call s:hi("MatchParen", s:nord8_gui, s:nord3_gui, s:nord8_term, s:nord3_term, "", "")
 call s:hi("NonText", s:nord2_gui, "", s:nord3_term, "", "", "")
 call s:hi("Normal", s:nord4_gui, s:nord0_gui, "NONE", "NONE", "", "")
@@ -219,7 +219,7 @@ endif
 "+--- Gutter ---+
 call s:hi("CursorColumn", "", s:nord1_gui, "NONE", s:nord1_term, "", "")
 if g:nord_cursor_line_number_background == 0
-  call s:hi("CursorLineNr", s:nord4_gui, s:nord0_gui, "NONE", "", "NONE", "")
+  call s:hi("CursorLineNr", s:nord4_gui, "", "NONE", "", "NONE", "")
 else
   call s:hi("CursorLineNr", s:nord4_gui, s:nord1_gui, "NONE", s:nord1_term, "NONE", "")
 endif