REP 149: Package Manifest Format Three Specification (#138)

* add draft for REP 149: Package Manifest Format Three Specification

* Update rep-0149.rst

* update based on feedback

* add information about dependencies for ROS 1 and ROS 2

* clone package format 2 schema to format 3

* update format 3 schema

* add condition attribute

* add section about to be defined environment variables

* mention need to update ROS 1 tools

* apply feedback