fix: setup script for Windows

[josefaidt]

Issue #, if available:

n/a

Description of changes:

• changes bash file to js
• adds setup script
• adds postinstall script
• executes setup in postinstall (mitigates changing the readme steps)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[timngyn]

Looking into why the build failed

[timngyn]

@josefaidt so I think it's failing because of the aws-exports.js file that gets created post install. I pulled the branch down, did yarn and then tried to do amplify pull and this was the error I got:

Failed to pull the backend.
🛑 Unexpected token 'export'
/Users/timngyn/Development/aws-amplify/pull-requests/docs/src/aws-exports.js:1
export default {}
^^^^^^

SyntaxError: Unexpected token 'export'
...
...

[katieklein]

@josefaidt - It looks like you've resolved the build issues, however there are merge conflicts - can you please resolve these? Thank you!