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,
}
```
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