Implement Github Action CI flow for 2GP

[ndrewr]

The 2nd-Generation Packaging paradigm has the "source of truth" living in source control itself as opposed to in a specific holy org.

We test and deploy via package installation, and we use CLI sfdx force:package api to do this vs the older method using sfdx force:mdapi commands.

See:
[Second Gen Packaging Overview(https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_dev2gp.htm)

and in particular:

Second Gen Packaging Workflow

A project's CI flow will need to know what kind of packaging paradigm is being employed, and call different commands or scripts in response.

1. Implement a way to configure or signal to a project that this is 1st or 2nd Gen

2. Implement CI flow based around 2GP using Github Actions

3. Update the Readme to explain how to setup and use

[ndrewr]

This series outlines a sample workflow around 2GP/Unlocked packages:
https://developer.salesforce.com/blogs/2018/06/working-with-modular-development-and-unlocked-packages-part-4.html

NOTE
Going thru the sample project's CircleCI config could be a good guide for Gitub Action-powered flow.

https://github.com/trailheadapps/easy-spaces/blob/master/.circleci/config.yml