Roadmap post: nested record types

[fhammerschmidt]

rescript-lang/rescript#7241

Outline:

• Why do we need this? Describe how it is especially useful for writing bindings where the nested record types tend to not be used outside of their parent record anyways.
• Describe how this is only syntactical sugar (behind the scenes there are still record types with their own names created)
• Add some real-world examples, could be just from a bindings library

PS: externals are not ready yet, so let's don't mention them at all for now: rescript-lang/rescript#7791