PS: Update AST hierachy after #100.

Author: MathiasVP

powershell/ql/lib/semmle/code/powershell/AssignmentStatement.qll

@@ -1,6 +1,6 @@
 import powershell
 
-class AssignStmt extends @assignment_statement, Stmt {
+class AssignStmt extends @assignment_statement, PipelineBase {
   override SourceLocation getLocation() { assignment_statement_location(this, result) }
 
   int getKind() { assignment_statement(this, result, _, _) }

powershell/ql/lib/semmle/code/powershell/CommandBase.qll

@@ -1,3 +1,3 @@
 import powershell
 
-class CmdBase extends @command_base, Stmt { }
+class CmdBase extends @command_base, Chainable { }