Fix type inferencing, syntax highlighting and code completion for `groovy-contracts`

[eric-milles]

Consider the following:

import groovy.contracts.*

@Invariant({ speed >= 0 })
class Rocket {
  int speed
  boolean started = true
  @Requires({ isStarted() && inc > 0 })
  @Ensures ({ isStarted() && old.speed < speed })
  int accelerate(int inc) { speed += inc }
}

Editor support for the condition expressions is pretty minimal.

[eric-milles]

Also supported by: apache/groovy@710de13