introduce RegExpInterpretation instead of RegexString, and move RegexTreeView.qll into a regexp folder

Author: erik-krogh

python/ql/lib/semmle/python/PrintAst.qll

@@ -7,7 +7,7 @@
  */
 
 import python
-import semmle.python.RegexTreeView
+import semmle.python.regexp.RegexTreeView
 import semmle.python.Yaml
 
 private newtype TPrintAstConfiguration = MkPrintAstConfiguration()