Add DeltaBundler's Flow types to flow config (#20953)

Summary:
This PR adds a declaration to `flow-github/metro.js` that fixes this error in `ci/circleci: analyze`:

```
Error ----------------------------------------------------- node_modules/metro-config/src/configTypes.flow.js.flow:18:27

Cannot resolve module `metro/src/DeltaBundler/types.flow.js`.

   18| import type {Module} from 'metro/src/DeltaBundler/types.flow.js';
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
Pull Request resolved: https://github.com/facebook/react-native/pull/20953

Differential Revision: D9642861

Pulled By: hramos

fbshipit-source-id: c73c546b1645a3df9bb5f541764135f8a0c76578