Whitelist Platforms

[dcramer]

Right now platforms are arbitrary, but that's not actually true in Sentry. We have strongly keyed names for the language support, and SDKs need to reflect that to maintain behavior.

We should create an internal whitelist of known language keys, and if a platform does not match that, we'll simply treat it as "other". It may we still store the raw value, but the system wouldn't recognize it outside of that.

As part of this we should also take care of #1329 and document the language keys and their reasoning.