Specification doesn't allow `end extension`

## Compiler version

`3.6.2`

## Minimized code

```scala
extension (system: Any)
  def foo: Any = ???
end extension
```

## Output

Compiles

## Expectation

The specification ([§1.2.2](https://scala-lang.org/files/archive/spec/3.4/01-lexical-syntax.html)) should be aligned with the implementation
