Legacy decorators depend on the class-properties transform to lower decorated
fields: a decorated field is emitted as an _initializerWarningHelper(...)
marker that class-properties turns into an _initializerDefineProperty call in
the constructor. When class-properties is enabled it lowers every field,
private element and method of the class. When it is not enabled (e.g.
targeting an engine with native class fields), the marker survives and throws at
runtime.

This makes legacy decorators work without the class-properties transform, along
the lines @nicolo-ribaudo suggested in the issue:

• A decorated instance field is lowered into the constructor (via
  initializerDefineProperty), so a decorator that replaces the field with a
  prototype accessor keeps working — a native field initializer would create an
  own data property that shadows the prototype getter. This is covered by the
  new getter-decorator fixture.
• Native field initializers run before the constructor body, so an undecorated
  field that follows a decorated one would otherwise initialize too early. To
  preserve the observable initialization order, every instance field after the
  first decorated one (public and private) is relocated into the constructor in
  source order. Fields before the first decorated field, methods and static
  members stay as native class syntax. When a computed key would need to move
  (its evaluation time would change), we keep the class-properties handshake.
• When the class-properties transform is enabled, the existing output and
  field-initialization order are preserved unchanged (the marker/handshake path
  is kept). Detection uses the class-features fields feature flag, so with
  @babel/preset-env the field lowering still happens automatically when the
  targets need it.

Before / after

class Example {
  before = 1;
  @dec decorated = 2;
  after = 3;
  #trailingPrivate = 4;
  method() {}
}

With @babel/plugin-proposal-decorators (legacy) and no class-properties, this
previously emitted the throwing _initializerWarningHelper. It now produces
working output that keeps the members that can stay native (fields before the
decorator, the method, the private field declaration) as class syntax, while
initializing the decorated field and the fields that follow it from the
constructor in source order:

let Example = (_class = class Example {
  constructor() {
    babelHelpers.initializerDefineProperty(this, "decorated", _descriptor, this);
    babelHelpers.defineProperty(this, "after", 3);
    this.#trailingPrivate = 4;
  }
  before = 1;
  #trailingPrivate;
  method() {}
}, _descriptor = babelHelpers.applyDecoratedDescriptor(/* ... */), _class);

An existing fixture in @babel/plugin-transform-typescript
(class/abstract-class-decorated-parameter, a real mobx @observable field)
went from emitting the throwing _initializerWarningHelper to working
constructor initialization.

Disclosure: this change was written with AI assistance (see the Co-Authored-By
trailer on the commits). I've reviewed and can walk through every line.