[Static extensions] Should we allow static extensions to be accessed on any named type?

The current proposal (#3835) for static extensions only allows static members and constructors to be accessed only on "class-like" types (that is classes, mixin classes, mixins, enums, and extension types). In principle, we could relax this restriction, either in the case that the `on` type is a typedef (see #4052) or in the case that the receiver is a typedef name. For example, we could allow:

```dart
typedef Pair = (T, T);

extension E on Pair {
factory Pair.twice(T x) => (x, x);
}
void test() {
var (x, y) = Pair.twice(3);
assert(3 == y);
assert(3 == x);
}
```

If so, do we attach the extension to the name `Pair` or to the underlying type? That is, does the following work?

```dart
typedef Pair = (T, T);

extension E on (T, T) {
factory Pair.twice(T x) => (x, x);
}
void test() {
var (x, y) = Pair.twice(3);
assert(3 == y);
assert(3 == x);
}
```

cc @dart-lang/language-team

Start by reading the current static-extension proposal in issue #3835 and the typedef discussion in issue #4052. Compare the two typedef examples in this issue and determine whether extensions attach to the typedef name or its underlying type. Done means the language team has resolved the design question and documented the resulting rule.

Written by the indexing model from the issue text.