feat(dashboard): Setup parallel V2 routes (#6691)

**What**
- Moves all current route definitions to `v1.tsx`
- Creates a new `v2.tsx` file for future V2 routes.
- Updates RouteProvider to serve V1 or V2 routes based on if `MEDUSA_V2` is set.

**How to use it**
When working on V2 domains you should set `MEDUSA_V2=true` in `/packages/admin-next/dashboard/.env`. We can't use the Feature Flag in the Medusa project since there are breaking changes in the `/admin/store` endpoint between the two versions, so it needs to be managed manually.

**For people implementing V2 domains**
To add a new route you should:

- Add the route component in `/v2-routes`
- Define which path will render it in `v2.tsx`.

I would prefer that we don't try to implement a lot of conditional code that will make the codebase unnecessarily complex. So if I was to implement the V2 login page, I would copy/past the v1 login route into `v2-routes`, make the required changes, eg. replace `useAdminLogin` with `useAdminCustomPost(...)`, instead of trying to force the current V1 Login page to work with both APIs.

There will hopefully be a lot of components/sections from each domain that is re-usable between versions. As an example if you are working on the Product details page for V2 you might want to reuse the `product-general-section.tsx` file, as it works without having to make any modifications. In that case I propose that we create a `modules/product` folder and move the component in there which we can them import into both the v1 and v2 version of the route.

packages/admin-next/dashboard/src/vite-env.d.ts

@@ -1,7 +1,8 @@
-/// <reference types="vite/client" />
+// / <reference types="vite/client" />
 
 interface ImportMetaEnv {
   readonly MEDUSA_ADMIN_BACKEND_URL: string
+  readonly MEDUSA_V2: boolean
 }
 
 interface ImportMeta {