Add a rule to determine which extension types are always-exhaustive

[eernstg]

See #3524 (comment) for background information.

This PR adds a rule to the extension type feature specification, specifying that an extension type is always-exhaustive if and only if its instantiated representation type is always-exhaustive (note that this rule may be applied repeatedly because the representation type could itself be an extension type).

Example:

extension type SmartBool(bool b) {}

String foo(SmartBool sb) {
  switch (sb) { // This switch must be exhaustive.
    case SmartBool(b: true): return 'True';
    case SmartBool(b: false): return 'False';
  }
}

It is my understanding that this behavior has already been implemented (perhaps it was implemented like that all the time), so there should not be any implementation effort associated with this spec change.

We have (or will have) language tests about this as well, so it should be detected if it is not true that this behavior is implemented already.

[eernstg]

@sgrekhov, I think we already acted on this change when the underlying language design question was decided? The actual text in the feature specification should not be controversial, and it should not change anything, but it would be great if you can double check the situation when this lands.

[sgrekhov]

I'll check it