Extract the MV3-relevant scripts to the SDK
Background / User story
We have the scripts in "scripts" directory that were meant to be used by the consumers for subscription-related actions:
- update (generate the subscription list file)
- fetch (download to the file system)
- merge (merge the subscriptions list files to one)
- convert
Most of it was introduced for the upcoming MV3 changes.
Now it became clear that some functionality is:
- consumed only through webext-sdk
- adds additional complexity of wrapping and exposing it if having here
Some unrelated scripts must leave as-is.
What to change
We need to leave only the scripts that are meaningful in the context of core and MV2. This might assume a separating of "updateSubscriptions" script.