fix(build): support nested assetsDir

Author: brc-dd

docs/reference/site-config.md

@@ -360,7 +360,7 @@ export default {
 - Type: `string`
 - Default: `assets`
 
-The directory for assets files. See also: [assetsDir](https://vitejs.dev/config/build-options.html#build-assetsdir).
+Specify the directory to nest generated assets under. The path should be inside [`outDir`](#outdir) and is resolved relative to it.
 
 ```ts
 export default {

src/node/config.ts

@@ -74,7 +74,7 @@ export async function resolveConfig(
   const site = await resolveSiteData(root, userConfig)
   const srcDir = normalizePath(path.resolve(root, userConfig.srcDir || '.'))
   const assetsDir = userConfig.assetsDir
-    ? userConfig.assetsDir.replace(/\//g, '')
+    ? userConfig.assetsDir.replace(/(^\.?\/|\/$)/g, '')
     : 'assets'
   const outDir = userConfig.outDir
     ? normalizePath(path.resolve(root, userConfig.outDir))