feat: add generateOnlyEnums option to typescript

[c0m1t]

generateOnlyEnums was added to typescript plugin options.
This allows typescript plugin to generate only enums.

Our team use relay package. relay-compiler generates Query, Mutation and fragment types based on schema.graphql file. So other types this library generates are not useful to us, except enums.

The problem is whenever we have a type that uses an enum it generates that type again, I mean there could be definition of an enum in 20 different files. We always liked to have a single source of truth for enums. We even wrote a small package relay-enum-generator. So we would really appreciate it if there were an option in your package to make this possible. I suppose other teams using relay probably have this issue.

[dotansimha]

Thank you @c0m1t !
We recently merged this: #3760 this also does that, and generates only enums ;)