feat(admin-sdk,admin-bundler,admin-shared,medusa): Restructure admin packages (#8988)

**What**
- Renames /admin-next -> /admin
- Renames @medusajs/admin-sdk -> @medusajs/admin-bundler
- Creates a new package called @medusajs/admin-sdk that will hold all tooling relevant to creating admin extensions. This is currently `defineRouteConfig` and `defineWidgetConfig`, but will eventually also export methods for adding custom fields, register translation, etc. 
  - cc: @shahednasser we should update the examples in the docs so these functions are imported from `@medusajs/admin-sdk`. People will also need to install the package in their project, as it's no longer a transient dependency.
  - cc: @olivermrbl we might want to publish a changelog when this is merged, as it is a breaking change, and will require people to import the `defineXConfig` from the new package instead of `@medusajs/admin-shared`.
- Updates CODEOWNERS so /admin packages does not require a review from the UI team.

packages/admin/dashboard/vite.config.mts

@@ -0,0 +1,36 @@
+import inject from "@medusajs/admin-vite-plugin"
+import react from "@vitejs/plugin-react"
+import { defineConfig, loadEnv } from "vite"
+
+// https://vitejs.dev/config/
+export default defineConfig(({ mode }) => {
+  const env = loadEnv(mode, process.cwd())
+
+  const BASE = env.VITE_MEDUSA_BASE || "/"
+  const BACKEND_URL = env.VITE_MEDUSA_BACKEND_URL || "http://localhost:9000"
+  const STOREFRONT_URL =
+    env.VITE_MEDUSA_STOREFRONT_URL || "http://localhost:8000"
+
+  /**
+   * Add this to your .env file to specify the project to load admin extensions from.
+   */
+  const MEDUSA_PROJECT = env.VITE_MEDUSA_PROJECT || null
+  const sources = MEDUSA_PROJECT ? [MEDUSA_PROJECT] : []
+
+  return {
+    plugins: [
+      react(),
+      inject({
+        sources,
+      }),
+    ],
+    define: {
+      __BASE__: JSON.stringify(BASE),
+      __BACKEND_URL__: JSON.stringify(BACKEND_URL),
+      __STOREFRONT_URL__: JSON.stringify(STOREFRONT_URL),
+    },
+    server: {
+      open: true,
+    },
+  }
+})