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.
Adds the mechanism to partially resolve an entire script (aka every underlying variable in ytdl-sub). This will be used in the upcoming `inspect` sub-command where it will dump a subscription's raw contents at multiple levels of resolution.
Potentially useful to optimize script execution as well.
