new groovy-astbuilder module in Groovy 4

[Paul King <pa...@asert.com.au>]

I was going to work on GROOVY-4694 which is to move the AstBuilder* global
transform into its own module. This would move the transform and related
org.codehaus.groovy.ast.builder classes into a groovy-astbuilder module.

Given that macros are our preferred option for most scenarios where the old
ast builder functionality would be used, I was going to make the module
optional. This should speed up compilation for the majority of cases where
that global transform isn't required. Let me know if you see any problems.

Cheers, Paul.

* not related to the antlr4 helper class of the same name.