fix(build): missing babel helpers, true esm modules, simplify

[pascalduez]

Hi,

• The Rollup Flow plugin is unnecessary, this is already handled by Babel and the babel-preset-flow.
• The current esm build is still transformed to commonjs, it's actually built two times.
  So this simplify a bit things, and leverage Rollup for what it's good at (handling multiple builds).
• The source files with Flow annotations should be output into the cjs dir, flow-bin is a node tool.
• Don't exlude babel helpers from final bundle. Fixes Babel helpers missing #289 babelHelpers is not defined #294
  Only the typeof helper is used, so including it is amost costless. Including all helpers would,
  or requiring the users to install the plugin and including it in their code.

[vvo]

Nice work @pascalduez, notify us when ready to go

[ghost]

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Happy coding!

[pascalduez]

@vvo I think it's good to go.

[ArtieReus]

This fix works for me. The given example from #294

reactElementToJSXString(<p style={{ color: 'red' }}>Test</p>)

goes through without errors.

[armandabric]

👍