perf!: client / server subpaths (#448)

* perf!: client / server subpaths

* chore: keep `createHeadCore` export, add `unhead/legacy`

Author: harlan-zw

Diff for: examples/vite-ssr-react/src/entry-server.jsx

@@ -1,6 +1,6 @@
 import ReactDOMServer from 'react-dom/server'
 import { StaticRouter } from 'react-router-dom/server'
-import { createHead } from '../../../packages/unhead/src/createHead'
+import { createHead } from 'unhead/server'
 import { App } from './App'
 
 createHead()

Diff for: packages/addons/test/infer-seo-meta.test.ts

@@ -1,5 +1,5 @@
 import { renderSSRHead } from '@unhead/ssr'
-import { createHead } from 'unhead'
+import { createHead } from 'unhead/server'
 import { describe, it } from 'vitest'
 import { InferSeoMetaPlugin } from '../src/plugins/inferSeoMetaPlugin'