Remove everything that is common with WebGL 1.0

After discussion with Ken, we agreed that for now the WebGL 2.0
spec should start by only describing the differences from WebGL 1.0.

At the moment, most sections are placeholders. New content includes:

- two new enums in "Types"
- new attribute in WebGLContextAttributes
- getParameter types under "Setting and getting state"
- everything from the previous commits