Migrate bridge_prefs.js to intermediate file format

rdsys wants to read the most up-to-date bridge lines from the tor-browser-build repo, rather than having duplicate sources of truth. Currently, bridge lines are deployed to the tor-browser prefs by the bridge_prefs.js file.

Rather than having rdsys parse this javascript file, we should instead store the bridge lines in some intermediate data format (json, csv, etc) that can be easily parsed by rdsys and some new script in the tor-browser project which will generate bridge_prefs.js at build time.