Haxe 4 documentation

[markknol]

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
• [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
• [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