Add [[cmov]] and [[unroll]] with a count of unrolls to perform on top of GL_EXT_control_flow_attributes

Id would be good to ensure that certain `?` expressions dont compile into an if-statement.

Also manually specifying the number of unrolls that can be performed would be nice.

Start by reviewing the GLSL specification and the GL_EXT_control_flow_attributes extension referenced in the title. Clarify the intended semantics for [[cmov]] and for specifying an unroll count, including how completion would be verified in the specification and related extension text.

Written by the indexing model from the issue text.