Deal with _hx_new

[Simn]

3 steps if all goes well:

1. Rename _new to _hx_new
2. Add CfAbstractConstructor
3. Add a_constructor

Closes #11994

[Simn]

There are now only 3 occurrences of the "_hx_new" string in the codebase:

1. The one from the actual renaming.
2. The one that checks the name for is_abstract_constructor. This could be replaced with some metadata but I don't think it's worth it.
3. Some field_name case in inheritDoc.ml that I don't really understand, so I left it alone.

[kLabz]

Checking what's wrong with CsSafeTypeBuilding here... 🤔

[kLabz]

Checking what's wrong with CsSafeTypeBuilding here... 🤔

12 |  @:persistent static var generated = new Map<String, Bool>();
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^
   | haxe.ds.Map does not have a constructor

Uh?

[Simn]

I was hoping that something would fail because I'm not dealing with the new a_constructor in the hxb reader yet. With any luck it's that.

[kLabz]

Good thing this is done before preview, that way we don't have to bump hxb version 😁