Expands variable validation to also include support for partial resolution.
In short, this will partially execute script code until it hits unresolved runtime variables, and store that as the new script representation. This functionality will allow for the upcoming `inspect` command, which will show users their script code in action without having to dry-run.
Implements https://github.com/jmbannon/ytdl-sub/issues/1182
Adds the ability to change the `keep_files_date_eval` (traditionally upload_date_standardized) to any date, so long as its in the form of YYYY-MM-DD.
Will be able to use this to have episodes get deleted based on their release_date or epoch_date, instead of their upload_date.
