expose FLOW_TRANSFORMS on relay-compiler

Related to a few issues with had on relay-compiler-typescript-plugin

https://github.com/relay-tools/relay-compiler-language-typescript/pull/109

I think most of the time, the Typescript transforms will be the same as Flow transforms

so it makes sense to export FLOW_TRANSFORMS, so plugin languages could be synced

Start by locating the relay-compiler entry point that defines FLOW_TRANSFORMS and inspect how the relay-compiler-typescript-plugin consumes or mirrors transforms, using the linked pull request for context. Done means FLOW_TRANSFORMS is exported in a way plugin languages can use to stay synchronized, with relevant compiler tests updated or added.

Written by the indexing model from the issue text.