[JS migration] New source and unit test files

[EmilianoSanchez]

JS SDK

What did you accomplish?

Added source files. SplitFactory function, for Node and Browser, is defined as in the new SDKs (e.g., RN, Browser) based on JS-commons:

export function SplitFactory(config) {
  const settings = settingsFactory(config);
  const modules = getModules(settings);
  return sdkFactory(modules);
}

How do we test the changes introduced in this PR?

Added new unit tests. Most validation is covered in JS-commons and by E2E tests.

Extra Notes