Fix #127 - Create "declarative_net_request" part which should be included in Manifest file

Added the missing "convert" part from the core. In the core scripts/text2dnr accepts in/out file. In the tooling we work with directories mostly, so scripts/convertSubscriptions.js here fills that gap. Added the scripts usage in README.

Also the script "scripts/generateSubscriptionsFragment.js" generated the fragment. It's used from webpack config to generate a valid manifest.json with the fragment.

By default the rulesets are disabled, they need to be manually enabled for testing (see dist/test-mv3/manifest.json declarativeNetRequest/rule_resources[N].enabled).

I've tested it and no manual actions were required (except editing of subscriptions list to test pages subscription https://testpages.adblockplus.org/en/abp-testcase-subscription.txt).