Refs EE-344, EE-102 - Merge test-mv2 and test-mv2-custom extensions, unify custom-subscriptions.json

Main changes:

• Removes the test-mv2-custom extension. Custom subscriptions are moved to the test-mv2 extension
• Keeps dist/ewe-api.js as it was (no custom subscriptions), while dist/ewe-subs/ewe-api.js is now used by the test-mv2 extension.
• Merges test/custom-subscriptions.json and test/scripts/custom-mv3-subscriptions.json into test/scripts/custom-subscriptions.json. That removes the remaining subscriptions duplication in the project.
• Building the extension now creates separate scriptsOutput/custom-mv2-subscriptions.json and scriptsOutput/custom-mv3-subscriptions.json to be used accordingly:
  • custom-mv2-subscriptions.json: The result of merging test/scripts/custom-subscriptions.json and core/data/subscriptions.json. Created with npm run subs-run-mv2.
  • custom-mv3-subscriptions.json: Created with npm run subs-run-mv3 (renaming the old npm run subs-run)