Haxe 4 documentation

We have to start making docs for when Haxe 4 is ready. This is a WIP of stuff the needs to be written/documented in the manual.

We have to add `\since{4.0}` to the new sections.

* [x] **final keyword**: [contribute here](https://github.com/HaxeFoundation/HaxeManual/pull/354)
* [x] **arrow function syntax**
* [x] **function type syntax**
* [x] **enum abstracts**
* [x] **inline calls** (btw still missing the override restriction?)
* [x] **extern keyword**
* [x] **xx.Syntax** for all [target details](https://haxe.org/manual/target-details.html)
* [x] **.platform.hx** file, with constraints (same directory)
* [x] **unicode**
* [x] **resolve changes**
* [ ] **ml plugin** docs / example:
https://github.com/HaxeFoundation/haxe/blob/development/std/eval/vm/Context.hx#L36