fix: always use image icon for image links, regardless of URL type

[twio142]

This PR fixes an issue where image links (![alt](...)) do not consistently display the image icon.

Currently, the plugin assigns icons based on the link target:

• Local image paths correctly use the image icon
• Image links pointing to web URLs (e.g. https://...) are treated as normal web links and get the web URL icon instead

I personally think that image links should always display the image icon, regardless of the URL type.

[MeanderingProgrammer]

This is intentional behavior, to prioritize custom icon definitions regardless of link type, image is just used as a fallback.

[twio142]

I think image links are essentially different to other lins. They rarely need to be manually opened, so the concrete protocol or domain are not that relevant. It's more important to be able to identify the images.

I use snacks.nvim to render images only when the cursor is on the image link. So it makes more sense to me to always have the image icon available, otherwise they look exactly the same as normal links.

Hope you could reconsider this.