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/tailwind.config.cjs

@@ -0,0 +1,28 @@
+import path from "path"
+
+const rootProject = path.join(
+  process.cwd(),
+  "../../apps/server/src/admin/**/*.{js,jsx,ts,tsx}"
+)
+
+// get the path of the dependency "@medusajs/ui"
+const medusaUI = path.join(
+  path.dirname(require.resolve("@medusajs/ui")),
+  "**/*.{js,jsx,ts,tsx}"
+)
+
+/** @type {import('tailwindcss').Config} */
+module.exports = {
+  presets: [require("@medusajs/ui-preset")],
+  content: [
+    "./index.html",
+    "./src/**/*.{js,ts,jsx,tsx}",
+    medusaUI,
+    rootProject,
+  ],
+  darkMode: "class",
+  theme: {
+    extend: {},
+  },
+  plugins: [],
+}