[Chore] Simplify the generation of the type definition files

React Strict DOM has a single file for Flow type definitions and a single file for TS type definitions.

The build step for this is fairly complex, however:

• We transform the entire folder of source files to individual .js.flow and .d.ts files using flow-api-translator.
• We then use a script to concatenate all the type files into a single file
• We then delete the folder

The script that concatenates the types from all the files is brittle and can break if we accidentally use the same type name in different files.

We should be able to simplify this process by using this Rollup-like bundler that understands Flow syntax.

We will need to wait for this bundler to be migrated to the hermes repo and open-sourced as a standalone package.

First check whether the referenced flow-bundler has been migrated to the hermes repo and released as a standalone package. Then compare its possible use with the current flow-api-translator, concatenation script, and folder-deletion build steps. Done means the generated Flow and TypeScript definitions remain correct without the brittle concatenation process.