This is a refactoring PR that paves the road to implement pzuraq/ecma262#12.

Previously we are searching the first field to inject initProto. In this PR we create a fieldInitializerAssignments queue, which will be prepended to the next available non-static field initializers, falling back to the class constructor. Though in this PR the fieldInitializerAssignments contains only the initProto call, it is designed to work with other assignments as well, such as the extra initializers introduced in pzuraq/ecma262#12.

For example, according to the updated spec, the input

class C {
  @dec #p = 1;
  #q = 2;
}

should be transformed to

class C {
  static {
    [_init_p, _extra_init_p] = _applyDecs(this, [[dec, 0, "p", o => o.#p, (o, v) => o.#p = v]], [], 0, _ => #q in _).e;
  }
  #p = _init_p(this, 1);
  #q = (_extra_init_p(this), 2);
}

as the extra initializers added by context.addInitializers() are now run after the field is created in the receiver class.

The test fixtures are updated because protoInit and the staticInit id are now declared before other generated identifiers. There are no behaviour changes introduced in this PR.