Add icons for providers

[tth05]

These options should be available:

• (if the suggestion has an icon, show both. otherwise, show provider icon)
• (if the suggestion has an icon, show that. otherwise, show provider icon)
• (always show the provider icon, even if the suggestion has an icon)
• (no icons at all. for those who prefer a minimalist look)

(Images taken from conversation here https://github.com/tth05/obsidian-completr/pull/44#issuecomment-1334818534)