`default get` / `default set` for properties

While we might add https://github.com/HaxeFoundation/haxe-evolution/pull/96 in 5.0 (?), current property syntax could be enhanced with a new modifier which would be equivalent to `public var propertyWithField:T { get; set; }`:

```haxe
public var propertyWithField(default get, default set):T;
```

This would be equivalent to:

```haxe
@:isVar
public var propertyWithField(get, set):T;
function get_propertyWithField():T return propertyWithField;
function set_propertyWithField(val:T):T return propertyWithField = val;
```

Which allows child classes to override getter/setter.

---

Also while we're at it, maybe `init` like [in C#](https://learn.microsoft.com/en-us/dotnet/csharp/properties#init-only) would be a nice addition now that we have `final` fields with similar behavior :thinking:

Start by reviewing the current property syntax and haxe-evolution pull request #96, which the issue references. Done means defining and implementing the proposed `default get` and `default set` behavior, with a decision on whether C#-style `init` belongs in the same change.

Written by the indexing model from the issue text.