SI-7605 Deprecate procedure syntax

This commit covers three cases:
- constructor definitions      (def this {...})
- concrete method definitions  (def foo {...})
- abstract method declarations (def foo)

The deprecation is currently hidden behind -Xfuture pending IDE support
for migrating users from procedures to methods.
Merge pull request #3076 from soc/SI-7605-deprecate-procedures

SI-7605 Deprecate procedure syntax