Adding Github Actions and eslint changes for ask-sdk-core

[Shreyas-vgr]

Enabling Github Actions

Description

• Enabling the usage of Github Actions for unit test validations on packages [ask-sdk-controls, ask-sdk-core].
• Also migrate to use eslint on ask-sdk-core along with eslint issues fixed other packages will be done on other PRs.

Testing

• All platforms [ubuntu-latest, macos-latest, windows-latest] passed on node versions 10 and 12
• Test results from my forked repo: https://github.com/Shreyas-vgr/alexa-skills-kit-sdk-for-nodejs/actions/runs/206874139/workflow

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Docs(Add new document content)
• [ ] Translate Docs(Translate document content)

Checklist

• [x] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have read the README document
• [ ] I have added tests to cover my changes
• [x] All new and existing tests passed
• [x] My commit message follows Conventional Commit Guideline

License

• [x] I confirm that this pull request can be released under the Apache 2 license