fix(runtime-core): ensure props definition objects are not mutated during props normalization (close: #6915)

[LinusBorg]

During Props normalization, we add a couple of flags to the definition object for internal purposes, so this:

{
  type: String
}

becomes this:

{
  type: String,
  0: true,
  1: false
}

The problem is that we do this to the original object provided by the developer in the component's options. This can lead to issues when the developer re-uses these objects in different places.

This PR makes a shallow copy of that object before adding those flags, so the original definition stays untouched.

close: #6915