Expose Astro's trailingSlash config to plugins (ctx.site.trailingSlash)

[bimsonz]

Plugins that build absolute URLs (sitemaps, canonical, hreflang) need to know the site's URL convention, but the plugin context (ctx.site) only carries name, url, and locale. So a plugin has to hardcode a trailing slash.

That's fine for Astro's default, but wrong for a site configured with trailingSlash: 'never'. It's especially a problem for headless setups, where the front-end serves bare URLs (like /blog/post) and the plugin's sitemap then advertises /blog/post/, which the site doesn't serve.

Proposal: expose Astro's trailingSlash on ctx.site, riding the same path the i18n config already uses (astro:config:setup to virtual:emdash/config to readSiteInfo to SiteInfo). It's optional and defaults to 'ignore' (Astro's default), so nothing changes for existing sites.

I hit this building the emdash-plugin-seo sitemap feed for a headless site. Happy to be told if there's a better approach or if it needs shaping first.

Draft implementation: #2122 (core) and jdevalk/emdash-plugin-seo#2 (the consuming plugin).