Expose SafeEventEmitterProvider as a value, not just a type

[mcmire]

This is so that we can use this to create a fake provider object, as in the network clients in NetworkController (see #1116, packages/network-controller/tests/fake-provider-engine.ts, for the original source)

[MajorLift]

Closed by #1738.

Relevant comments:

• eth-json-rpc-provider migration - Integration into packages/ #1738 (comment)
• eth-json-rpc-provider migration - Integration into packages/ #1738 (comment)
• eth-json-rpc-provider migration - Integration into packages/ #1738 (comment)