Add a “shared” directory to the api folder

[qFlensT]

Briefly: create a shared directory that will duplicate the route for the path .../admin/* and .../store/*.

Currently, we have the following basic structure in the api folder:

.
└── src/
    └── api/
        ├── admin
        └── store

But with this approach, there is a problem that arises when we want an HTTP method to be available in both admin and store at once (let's say by calling HTTP method GET).

We can use the following approach:

src/
└── api/
    ├── shared/
    │   └── routes/
    │       └── product/
    │           └── index.ts //declare GET method here
    ├── admin/
    │   └── product/
    │       └── route.ts //reexport GET method from `shared`
    └── store/
        └── product/
            └── route.ts //reexport GET method from `shared`

But we still have to duplicate the middleware code and make an extra reexport. This is not serious, but it would be ideal to be able to declare route in the shared directory once and apply other settings like those set in middleware.ts and not use reexport. Plus, this approach will get official support, and a person who comes to a project where medusa.js will understand what this directory is used for.

[qFlensT]

Sorry, didn't notice that middleware supports regexp in matcher