feature: highlight configuration for startup section on dashboard

[smitropoulos]

Did you check the docs?

• I have read all the snacks.nvim docs

Is your feature request related to a problem? Please describe.

I want a clearer look on the dashboard and I think I am unable to configure the highlights on the startup section

Describe the solution you'd like

I would love to be able to apply highlights on the startup section like e.g. key

Describe alternatives you've considered

Tried to apply startup = { "%s", align = "center", hl = "normal" }, and then saw the source code, I think the highlights are hardcoded

Additional context

No response