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/admin-sdk/src/config/index.ts

@@ -0,0 +1,2 @@
+export * from "./types"
+export * from "./utils"

packages/admin/admin-sdk/src/config/types.ts

@@ -0,0 +1,11 @@
+import type { InjectionZone } from "@medusajs/admin-shared"
+import type { ComponentType } from "react"
+
+export type WidgetConfig = {
+  zone: InjectionZone | InjectionZone[]
+}
+
+export type RouteConfig = {
+  label?: string
+  icon?: ComponentType
+}

packages/admin/admin-sdk/src/config/utils.ts

@@ -0,0 +1,37 @@
+import { RouteConfig, WidgetConfig } from "./types"
+
+function createConfigHelper<TConfig extends Record<string, unknown>>(
+  config: TConfig
+): TConfig {
+  return {
+    ...config,
+    /**
+     * This property is required to allow the config to be exported,
+     * while still allowing HMR to work correctly.
+     *
+     * It tricks Fast Refresh into thinking that the config is a React component,
+     * which allows it to be updated without a full page reload.
+     */
+    $$typeof: Symbol.for("react.memo"),
+  }
+}
+
+/**
+ * Define a widget configuration.
+ *
+ * @param config The widget configuration.
+ * @returns The widget configuration.
+ */
+export function defineWidgetConfig(config: WidgetConfig) {
+  return createConfigHelper(config)
+}
+
+/**
+ * Define a route configuration.
+ *
+ * @param config The route configuration.
+ * @returns The route configuration.
+ */
+export function defineRouteConfig(config: RouteConfig) {
+  return createConfigHelper(config)
+}

packages/admin/admin-sdk/src/index.ts

@@ -0,0 +1 @@
+export * from "./config"