Add vite-plugin-sitemap

[pyyupsk]

Checklist

• [x] Title as described.
• [x] Make sure you put things in the right category.
• [x] The description of your item should be a sentence with less than 24 words.
• [x] Avoid using links and parentheses in description.
• [x] Omit unnecessary words already provided in the context (e.g. for Vite, a Vite plugin).
• [x] Use proper case for terms (e.g. GitHub, TypeScript, ESLint, etc.)
• [x] When mentioning tools, omit versions whenever possible (e.g. use TypeScript instead of TypeScript 4.x, but keep Vue 2 and Vue 3 as they're incompatible).
• [x] When mentioning package names, use quotes whenever possible.
• [x] Always add your items to the end of a list.

Plugins/Tools

• [x] The plugin/tool is working with Vite 2.x and onward.
• [x] The project is Open Source.
• [x] The project follows the Vite Plugins Conventions.
• [x] The plugin uses Vite-specific hooks and can't be implemented as a Compatible Rollup Plugin.
• [ ] The repo should be at least 30 days old.
• [x] The documentation is in English.
• [x] The project is active and maintained (inactive projects for longer 6 months will be removed without further notice).
• [x] The project accepts contributions.
• [x] Not a commercial product.