Add Core & Legacy Composer ScriptHandlers

This is for use in https://github.com/ezsystems/ezpublish-community/pull/34
Allows legacy assets to be symlinked and prod assets to be dumped as part of
composer install / update command to reduce amounts of steps needed for install.

Todo:
- [x] Add change as suggested by PS
- [x] Rebase with issue number in commit + PR title when done in ezsystems/ezpublish-community#34