feat: add plugin-icon

[Mister-Hope]

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[Mister-Hope]

@pengzhanbo Please review this and see if you can accept to use this plugin in your theme. Our goal is to generate a usable icon plugin across themes.

I don't think adding full package support for iconify is needed.

1. The full package is too large, and it's fast enough to access icons from iconify apis.
2. Icon is definitely NOT important for SSR and SEO.

So I would suggest using the recommend way that iconify icon suggest, rather than supporting https://theme-plume.vuejs.press/guide/features/icon/#%E5%8A%A0%E8%BD%BD%E5%9B%BE%E6%A0%87

Note: We can support iconify-icon package as well, but it has little benefit as users are already allowed to use any CDN links or even self host the js file and specific iconify type manually.

[Mister-Hope]

Pinging @recoluan for your theme-reco as well

[recoluan]

Pinging @recoluan for your theme-reco as well

Good job!

[pengzhanbo]

@Mister-Hope First of all, this plugin is great, but I have reservations about fully loading icon resources via CDN. In my practical application scenarios, as well as feedback from some users, it is quite common to have completely isolated external network connections within corporate intranets, which makes it impossible to load icon resources correctly in such cases. On the other hand, when there are a large number of icons present in the visible area of the site, it triggers a significant number of icon requests. Due to the browser's limitations on concurrent requests, this can lead to frequent UI flickering.

[Mister-Hope]

I decide to add this first. The next target is to support markdown grammar with it.

Then we can see if we can add built-in icons support for iconify.

For the grammar, I would prefer ::icon-name:: instead of :[icon-name]:, any reason that you choose :[xxx]: @pengzhanbo ?

[pengzhanbo]

@Mister-Hope
::icon-name lacks a clear ending boundary, and icons often need to support settings for color, size, etc. Without a clear boundary, it is difficult to determine when to end.

:[xx]: references the emoji format :xxx:, allowing for more content to be written within []. This can be expanded to the syntax :[icon-name size/color]: to support settings for size and color.