scripts validation, pre-publish

List of checks to run and block users trying to save/publish a script with illegal parts. I believe these validators should be checked over the YAML, after clicking on `publish`(https://github.com/hasadna/reportit-scripts/issues/15) , showing an explaination for the deploy failure, when relevant:

1. user is trying to save the value to a variable that is never being called/mentioned later.
2. user is trying to save a value to a constant variable (report field) name that does not exist.
3. script includes a goto points to a nonexisting block name
4. user is trying to create a temporary variable name (represented on YAML with `_`) with a constant variable name (field record name).
5. user is trying to create a temporary variable name (represented on YAML with `_`) with a saved word: (i.e. 'do', 'goto', etc.).