Fix react-native build with hermes enabled on Windows

[Shywim]

This should fixes #693.

For now, we can't deploy a react-native release on Windows with hermes enabled because the script looks for an executable file without a .exe file extension, whereas on Windows, we need a .exe extension, throwing an ENOENT error.

Full error:

Error: spawn node_modules\hermesvm\win64-bin\hermes ENOENT
    at Process.ChildProcess._handle.onexit (internal/child_process.js:264:19)
    at onErrorNT (internal/child_process.js:456:16)
    at processTicksAndRejections (internal/process/task_queues.js:77:11)
Emitted 'error' event at:
    at Process.ChildProcess._handle.onexit (internal/child_process.js:270:12)
    at onErrorNT (internal/child_process.js:456:16)
    at processTicksAndRejections (internal/process/task_queues.js:77:11) {
  errno: 'ENOENT',
  code: 'ENOENT',
  syscall: 'spawn node_modules\\hermesvm\\win64-bin\\hermes',
  path: 'node_modules\\hermesvm\\win64-bin\\hermes',
  spawnargs: [
    '-emit-binary',
    '-out',
    'C:\\Users\\shywi\\AppData\\Local\\Temp\\code-push2020317-23456-1rdxgc2.ndt9\\CodePush\\index.android.bundle.hbc',      'C:\\Users\\shywi\\AppData\\Local\\Temp\\code-push2020317-23456-1rdxgc2.ndt9\\CodePush\\index.android.bundle',          '-w'
  ]
}

This PR add a function to look for platform specific executable file extension.

[anywherepilot]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[anywherepilot]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[anywherepilot]

Thank you very much for figuring out the problem and solving it @Shywim. I agree this solves #693. We'll release a new version with this code so you and everyone can benefit.