highlight: Added QuickFixLine highlight group

- Links to Search by default

screen.c: Combine CursorLine with QuickFixLine

- HLF_QFL takes priority over HLF_CUL

docs: Updated to mention QuickFixLine
runtime: Added QuickFixLine to nvimHLGroup
tests: QuickFixLine highlight

src/nvim/syntax.c

@@ -5902,6 +5902,7 @@ static char *highlight_init_both[] =
   "VertSplit    cterm=reverse gui=reverse",
   "WildMenu     ctermbg=Yellow ctermfg=Black guibg=Yellow guifg=Black",
   "default link EndOfBuffer NonText",
+  "default link QuickFixLine Search",
   NULL
 };