if expressions

You already introduced switch expressions. Are you planning to add if expressions?

Syntax examples:
```dart
var x = if (a > 0) return 1; else return 2;
```
```dart
var x = if (a > 0) {
return 1;
} else if (b < 10) {
return 2;
} else {
return 3;
}
```
Currently we have only `x ? y : z` operator, but it's not enough if we want more advanced branching.

Start by reviewing the existing switch expressions referenced in the issue and compare them with the proposed if-expression syntax examples. Clarify the language-design requirements, including branching and return behavior, before determining whether the proposal is complete.

Written by the indexing model from the issue text.