feature(image): Add an option to disable downloading images by url

[sergei-grechanik]

Did you check the docs?

• I have read all the snacks.nvim docs

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

Currently snacks.image tries to download images by url. This could pose a security risk, so it would be nice to have a way to disable it.

Describe the solution you'd like

Add an option to disable image downloading.

Describe alternatives you've considered

I guess I can remove wget and curl from PATH, but it's too much of a hassle.

Additional context

No response