`uintX` assignable to larger `intY`

### Clear and concise description of the problem

Should for example `uint8` be assignable to `int16`. Today we don't allow that. We could allow but this would need some special handling
[playground](https://typespec.io/playground?c=bW9kZWwgRm9vPFQgZXh0ZW5kcyBpbnQxNj57fQoKYWxpYXMgVGVzdCA9xSV1aW50OD47&e=%40typespec%2Fopenapi3&options=%7B%7D)

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [docs](https://typespec.io/docs/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.