No longer use "Tag" opting for: "Stratum" or nothing

[jasonvanderslice]

Background

It became clear after open sourcing (and some recent inner sourcing) we need to ensure using Stratum (and contributing) are more open/friendly to full stack implementations. Was outlined with #5 and implemented with #3

What & Why

1. Ensure that Stratum's core can handle more contexts, not just Browser + Analytics
   1. This matches Open Telemetry's reasoning and implementation, which is a very complimentary/inspirational thing
2. Replace uses of "Tag" with either "Stratum" or nothing
   1. Simplifies when talking/building in Stratum, also opens up the door for generic full stack events