refactor!: migrate plugin to oclif/core v2

[k80bowman]

Work item

Migrates the plugin to oclif/core v2. Also includes the following changes:

• updates the engine to Node 20
• updates the scripts
• updates dependencies (eslint, Typescript, etc)
• updates/adds tests
• updates examples
• adds test coverage reporting
• updates github workflows
• adds bin files for easier local testing

Testing

• pull down this branch and run yarn && yarn build
• Link the plugin to your CLI installation by running heroku plugins:link from the root directory of this repo
• Run heroku api GET apps/APP_NAME. You should see details about your app
• Using an app with a dyno, run printf '{"updates":[{"type":"web", "quantity":2}]}' | heroku api POST /apps/APP_NAME/formation. You should see that the quantity of web dynos for that app is now set to two.
  • I was not able to write a unit test for this usage, so it is good to test it manually for now.

[eablack]

Works great! Couple of minor comments, approving.