feat(crewllinks): add support of showing external links' favicon

[dodolalorc]

The showLinkFavicon option automatically displays the favicon of external websites before each external link. When enabled, all external links will show a small icon representing the linked website, making it easier for users to visually identify the source of the link. The default value is false.

This feature is useful for improving the recognizability and aesthetics of external links, and is often used together with externalLinkIcon.

There is a picture of my using in blogs：

[github-actions[bot]]

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[dodolalorc]

I think this is a cute style, and maybe it's a good optional choice as a link style.

[DeanLemans]

I think this is amazing, i have a really long list of exter sites and i have to add the svg manually each timd. I cant believe i never thought about fetching the favicon automatically. I hope this gets merged soon

[saberzero1]

I don't have any strong opinion on this, just that if there is a lot of links on a page it might get rate-limited (a bit of a link spammer myself)

cc @saberzero1 @jackyzha0 when you guys have time.

Are the icons cached after initial load? If there are ten different links to a domain, is it fetched once or ten times?

Also, keep in mind that this is disabled by default.

Perhaps fetching during build could be an option? Then adding to /static/externalIcons/<hostname> or something? Assuming we only fetch unique icons once, it shouldn't hit any rate limit. If that hits a rate limit during build, it will likely also hit one during live usage.

Just some thoughts. I like this feature.