flow check

TODO: add more details
Flow is a type-script like language from Facebook. See https://flow.org/
We run flow-check as part of the CI loop to validate type information.
You can run flow-check by running yarn lint from the root of the repo. You can also run it within sub-projects (eg packages\react-native-win32).
Flow-check can be configured by creating a .flowconfig file. See https://flow.org/en/docs/config/.
The main purpose of this file is to list which files to ignore, which is needed for platform-specific overrides (eg if we have a Dimensions.windows.js, flow gets confused between this and Dimensions.js, so we ignore Dimensions.js)