Allow `codeUnitAt` to be constant for constant strings

Let's take the example from https://dart.dev/language/branches#switch-expressions:

```dart
// Where slash, star, comma, semicolon, etc., are constant variables...
token = switch (charCode) {
slash || star || plus || minus => operator(charCode),
comma || semicolon => punctuation(charCode),
>= digit0 && <= digit9 => number(),
_ => throw FormatException('Invalid')
};
```

It assumes that `slash`, `star`, ... are constant variables, but how are they defined? I expect to be able to do:

```dart
const slash = '/'.codeUnitAt(0); // Error: Methods can't be invoked in constant expressions.
// ...
```

but I can't, and it's ugly and error-prone to find out what the char code for each of the characters are and put the integer in instead.

Start with issue #4146 and the linked Dart switch-expression example, then read the language specification's rules for constant expressions and method invocation. Determine the language-design changes needed for `codeUnitAt` on constant strings and document the resulting decision and scope.

Written by the indexing model from the issue text.