[Chore] Simplify the generation of the type definition files

[nmn]

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.

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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.

[necolas]

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

I don't think that has been the case for a while. But it would be good to go back to 1-file libdefs using the flow bundler at some point