[DX] Improve route requirements definition: allow to define requirements inline with placeholders itself

| Q                | A
| ---------------- | -----
| Bug report?      | no
| Feature request? | yes
| BC Break report? | no
| RFC?             | yes
| Symfony version  | 4.x

It would be great to inline requirements to pattern itself:
`/route/path/{parameter:\d+}` instead of `/route/path/{parameter}, requirements={{parameter: "\d+"}}`