Allow variable binding on String patterns

Nobody has claimed this yet.

I didn't find a duplicate for this issue, but GitHub's search is terrible, so this may be a duplicate.

I was thinking that we could allow variable bindings to String patterns. For instance:

```dart
switch (myRoute) {
'/books/${final id}/author' => 'The author of the book with ID $id',
'/books/${final id}' => 'Book with ID $id',
'/books' => 'Books list',
_ => 'Invalid route',
}
```

We could do it in every way that it's possible today to do with other patters, and if we could even extend it to support assignment.

```dart
final '/books/${bookId}' = myRoute; // We are sure that `myRoute` have the expected pattern. If not, a runtime error is thrown.
final id = int.parse(bookId);
```

The issue names no files, tests, or entry points. Start by reviewing Dart's existing pattern-matching rules and the String pattern examples in the issue; done would require an agreed design for variable bindings, assignments, and runtime behavior.

Written by the indexing model from the issue text.