Add explicit schema versions

[jaredlockhart]

It would be nice if the schemaVersion field explicitly declared what values it expects, maybe something like

  enum SchemaVersion {
    V1: "v1",
    V2: "v2"
  }

  export interface NimbusExperiment {
    schemaVersion: SchemaVersion,
    ...
  }

[jaredlockhart]

Actually I realized it's probably better to use the version of this repo's published package as the version string, ie "1.0.0" for the current version that way it's easy to correlate with the tags in the repo, so I'll close this.