Infer `function` 

Shall infer `def function` [^1] in a type definition context.

```nx
struct Foo {
  bar() => baz()
}

# Equivalent of:
struct Foo {
  def function bar() {
    baz()
  }
}
```

[^1]: `foo()` → (`function foo()` ≡ `def foo()`) → `def function foo()`; `decl foo()` → `decl function foo()`; `impl foo()` → `impl function foo()`.