Fix: Model with spread indexer shouldn't validate explicit properites

[timotheeguerin]

This was causing errors in doing things like this

model Info {
  age: int32;
  ...Record<string>;
}

model Bar<T extends Info> {}

model Test {
  t: Bar<{
    age: 123;
    other: "abc";
  }>;
}

[azure-sdk]

All changed packages have been documented.

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/prs/3163/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/prs/3163/