feat: add generatorLibrary options and allow faker to select

[MH4GF]

closes #90, related #92

What

Add a new option generateLibrary to make faker available in addition to the existing casual.

Context

casual dependents on the node API(e.g. fs) and cannot be made to work in the browser. ref: https://github.com/ardeois/graphql-codegen-typescript-mock-data/issues/90

Additional Information

To simplify the code, we also added a class that returns the mock value generated by the library.

[MH4GF]

✅ TODO

• [x] refactor to add CasualMockValueGenerator class
• [ ] Eliminate "casual" import from index.ts
• [x] add FakerMockValueGenerator class
• [x] add generateLibrary option and switching MockValueGenerator
• [x] add description of generateLibrary option to README

[ardeois]

@MH4GF all good, merging and closing the issue