Refactor Field Type organization

[jasonbahl]

Currently, the class-config.php file includes a massive switch statement and logic for all field types.

Each field should have it's own Class file, extending a generic AcfField class, which sets up how the field is mapped to the GraphQL Schema and how the fields resolve.

[jasonbahl]

Closed by #262.

Nice new folder/class structure introduced in v0.6.0

Before

The entire plugin was basically in 4 files.

After

(still not final, but close enough to close this issue as resolved by v0.6.0)