Refactor Environment variable collection

Right now, it's not terrible, but it's also not very elegant. Things like #34 and #32 (and #30) all led to some spaghetti. It'd be nice to refactor this entirely, understanding that eventually more sources could get added (Zoom, Slack, MS Teams, Skype, Google Hangouts, Google Meet, Google Calendar and other iCal-capable calendars, etc.). Ideally this should switch to a plugin architecture.