fix: generate multiple swift files

[dpilch]

Description of changes

Generate multiple Swift type files. The separate files are for each GraphQL statement file. The multiple type files need the names of the GraphQL statements files in order to generate with the correct names.

This leaves us a few options for the queries arg.

1. [fileContents: string, filename: string][]
2. Source[] (from graphql package).
3. Type union of the current type (string) and one of the two above options.

I chose string | Source[] for this PR, but I don't have strong feelings on the end direction.

Issue #, if available

#711

Description of how you validated changes

• Unit tests
• Manual tests

Checklist

• PR description included
• yarn test passes
• Tests are changed or added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.