At this moment, it appears that there are no readily available solutions for the issue at hand.
When developing a library and exporting your module with index.js, you can include index.d.ts and index.js.flow to address this.
If you are already using types in your library, creating definitions can be frustrating, but fortunately, there are helpful resources out there.
Flow has a tool called flow gen-flow-files, though it's not yet stable.
As for TypeScript, I am unsure if a similar solution exists, but considering its maturity, one can hope they have developed something.
Now onto the arduous task of converting definitions between the two languages. If you start with TypeScript, there are a few options available to convert to Flow (although I lack personal experience with any).
On the other hand, if you begin with Flow, there doesn't seem to be a clear method for converting to TypeScript.
To address this specific problem, I have initiated the creation of a boilerplate and playground.
https://github.com/idanilt/flow_ts_lib
Currently, this solution meets my requirements by allowing me to write a flow-based library that supports both TypeScript and Flow applications (with simple regex conversion from Flow to TS).
However, I aim to broaden its scope by including more use cases so that future library developers won't encounter this issue again.