Update optional-features.json

[achambers]

Adding no-implicit-route-model for emberjs/rfcs#970

TODO

• Modify this to default to true

[mansona]

The CI is failing with the message:

Unknown feature "no-implicit-route-model" found in config/optional-features.json

Does this mean that we can't merge this until the version of ember-source that defines this optional feature is actually released? I don't know if we've done any version guarding on optional-features before 🤔

[mansona]

Ah looking into it it's probably because @ember/optional-features hasn't been shipped. I'll work on get that released and this should start passing, and we should probably bump the version in the blueprint too to be safe ;)