Improve editor experience for type-checking extension scripts

[eric-milles]

STC extension scripts are executed with the following configuration:

import org.codehaus.groovy.ast.* // Groovy 4.0.6+
import org.codehaus.groovy.ast.expr.*
import static org.codehaus.groovy.ast.ClassHelper.*
import static org.codehaus.groovy.transform.stc.StaticTypeCheckingSupport.*

@groovy.transform.BaseScript org.codehaus.groovy.transform.stc.GroovyTypeCheckingExtensionSupport.TypeCheckingDSL delegate

In addition to this, TypeCheckingDSL#invokeMethod provides a series of isXxxExpression(Object) methods and a number of type-checking callback registration methods. None of these enhancements are presently exposed through the Groovy editor, making for a poor experience.

[eric-milles]