Turn --enable-base-browser in --with-base-browser-version

The browser version customization might be needed for all our browsers (Tor Browser that has it already, Base Browser, and Privacy Browser).

I think it doesn't make much sense to have duplicate code for it and conditional code in tor-browser-build. So, I'm more up for changing all BASE_BROWSER with BASE_BROWSER_VERSION, and replace --enable-base-browser with a --with-base-browser-version option that takes an argument.

At the moment, we exit with an error if the user doesn't define a Tor Browser version, should we do it also for Base Browser? Probably it's a good idea, since we enable/disable stuff if it's defined.

Should we change TOR_BROWSER_VERSION in TOR_BROWSER, and then define --enable-tor-browser in browser/config/mozconfigs/tor-browser and in browser/config/mozconfigs/tor-browser-android? Or should we keep TOR_BROWSER_VERSION as an alias of BASE_BROWSER_VERSION?