Improve `gyp-to-cmake` to handle "conditions"

[kraenhansen]

As per https://gyp.gsrc.io/docs/UserDocumentation.md

The top-level pieces in the .gyp file are as follows:
'conditions': A list of condition specifications that can modify the contents of the items in the global dictionary defined by this .gyp file based on the values of different variablwes. As implied by the above example, the most common use of a conditions section in the top-level dictionary is to add platform-specific targets to the targets list.

See https://github.com/callstackincubator/react-native-node-api-modules/blob/7b07561614010134c2e1490225cadde09d913b28/packages/gyp-to-cmake/src/transformer.ts#L63-L66