Types: Add method overload for transform & bundle functions

supersede #91

Here is a simpler version for functions overloads (like esbuild) that seems to be a valid flow syntax (But I'm not sure it would provide as much type information than TS).

I'm using | undefined because it very uncommon to use void in that case and creates false positive with eslint rules that uses type information.

Does not appear to work with flow.

What do you think about committing the current version of flow types, to allow TS types to evolve without the need for a script to translate them into flow types?