feature: Make dashboard sessions work without Lazy

[itpropro]

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Did you check the docs?

• [x] I have read all the snacks.nvim docs

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

I am using NixOS and managing nvim packages with nix. It would be nice if the function have_plugin could be extended to check in a more generic way or alternatively provide your own check function via option to validate if the plugin has loaded. Reference: https://github.com/folke/snacks.nvim/blob/bc0630e43be5699bb94dadc302c0d21615421d93/lua/snacks/dashboard.lua#L777

Describe the solution you'd like

A way to provide an alternative lua function to check if a plugin exists/is loaded.

Describe alternatives you've considered

Additional context

No response