Reorganise compiler defines

The current list of [compiler defines](https://haxe.org/manual/compiler-usage-flags.html) is a bit messy.

- [x] update the list according to https://github.com/HaxeFoundation/haxe/blob/development/src/core/define.ml - ideally this could be auto-generated?
- [ ] list target-specific defines in individual `Defines` subsections for that target
- [ ] link to relevant sections if any (e.g. the new `warn-var-shadowing` will link to the Expressions / blocks, where shadowing is explained + the flag is used in context)