fix(colorscheme): underline StatusLineNC with 'notermguicolors' #28810

Problem: statusline for non-active window can be hard to distinguish
  from normal text with 'notermguicolors'. It was set to use only bold
  text to find a balance between being not too similar to active
  statusline and normal text, and be supported in enough terminal
  emulators (if it does not support 'termguicolors' there is higher
  chance that it also does not support underline).

Solution: reconsider balance by placing more emphasis on making
  non-active statusline more distinguishable.
  This also results into tabline being shown with underline which
  aligns with "make more distinguishable" shift.
(cherry picked from commit 9b9f54e)

src/nvim/highlight_group.c

@@ -369,7 +369,7 @@ static const char *highlight_init_light[] = {
   "SpellLocal           guisp=NvimDarkGreen  gui=undercurl                   cterm=undercurl",
   "SpellRare            guisp=NvimDarkCyan   gui=undercurl                   cterm=undercurl",
   "StatusLine           guifg=NvimLightGrey3 guibg=NvimDarkGrey3             cterm=reverse",
-  "StatusLineNC         guifg=NvimDarkGrey3  guibg=NvimLightGrey3            cterm=bold",
+  "StatusLineNC         guifg=NvimDarkGrey3  guibg=NvimLightGrey3            cterm=bold,underline",
   "Title                guifg=NvimDarkGrey2                        gui=bold  cterm=bold",
   "Visual                                    guibg=NvimLightGrey4            ctermfg=15 ctermbg=0",
   "WarningMsg           guifg=NvimDarkYellow                                 ctermfg=3",
@@ -454,7 +454,7 @@ static const char *highlight_init_dark[] = {
   "SpellLocal           guisp=NvimLightGreen  gui=undercurl                 cterm=undercurl",
   "SpellRare            guisp=NvimLightCyan   gui=undercurl                 cterm=undercurl",
   "StatusLine           guifg=NvimDarkGrey3   guibg=NvimLightGrey3          cterm=reverse",
-  "StatusLineNC         guifg=NvimLightGrey3  guibg=NvimDarkGrey3           cterm=bold",
+  "StatusLineNC         guifg=NvimLightGrey3  guibg=NvimDarkGrey3           cterm=bold,underline",
   "Title                guifg=NvimLightGrey2                       gui=bold cterm=bold",
   "Visual                                     guibg=NvimDarkGrey4           ctermfg=0 ctermbg=15",
   "WarningMsg           guifg=NvimLightYellow                               ctermfg=11",