Feat: Displayer now supports highlights for separator and elements (#304)

Separator Example: When creating the displayer
```lua
local displayer = entry_display.create {
  separator = " ",
  separator_hl = 'Visual', -- New
  items = {
    { width = opts.bufnr_width },
    { width = 4 },
    { remaining = true },
  },
}
```

Elements Example: When submitting data
```lua
local hl_group = x == y then 'Normal' or 'Visual'

return displayer {
  entry.bufnr,
  { entry.indicator, hl_group },
  display_bufname .. ":" .. entry.lnum,
}
```

lua/telescope/make_entry.lua

@@ -329,7 +329,7 @@ function make_entry.gen_from_buffer(opts)
       bufnr = entry.bufnr,
       filename = bufname,
 
-      lnum = entry.info.lnum and entry.info.lnum or 1,
+      lnum = entry.info.lnum ~= 0 and entry.info.lnum or 1,
       indicator = indicator,
     }
   end