K2 Scripting: wrap script statements into blocks

(reuse anonymous initializers as block wrappers) so the top-level script
elements are all declarations now. Rename the property accordingly (
together with the previous commit).
It makes script more similar to the class and thus simplify e.g.
control flow analysis and resolve code.

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -494,10 +494,10 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
 
         script.configure {
             +name
-            +FieldList("declarations", statement, withReplace = true, useMutableOrEmpty = true).withTransform()
+            +declarations.withTransform().withReplace()
             +symbol("FirScriptSymbol")
-            +fieldList("parameters", variable, withReplace = false)
-            +fieldList(contextReceiver, useMutableOrEmpty = true)
+            +fieldList("parameters", property).withTransform()
+            +fieldList(contextReceiver, useMutableOrEmpty = true).withTransform()
             +field("resultPropertyName", nameType, nullable = true)
         }