Add isActive() session method

[carmelal]

Why are these changes introduced?

• Fixes port verifyToken from koa-shopify-auth #133
• Easier to see whether a session is active

What is this pull request doing?

• Adds method checking if a session is active
• Adds SessionInterface type to use instead of Session
• Moves session storage tests to correct directory level
• Layers auth exports

Type of change

• Minor: New feature (non-breaking change which adds functionality)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[paulomarg]

This is looking great! While we're at it, we should also export the SessionInterface type since that's what apps will need to implement their custom session storages with types. We should also update the docs once it's done to reflect that.

[paulomarg]

LGTM, I'm happy for this to go in after we deal with my comment.

[thecodepixi]

This looks great! I think you just need to move your changelog note up a bit, but otherwise approved!