Move core logic into new "core" package

[matejchalk]

Proposal

• all our core logic that's not Yargs-related lives in utils - too much?
• utils could be only for more generic helper functions, e.g. executeProcess or Markdown formatters
• core logic like collect and executePlugin could live in core package

Motivation

• core package should be for programmatic usage of our core logic (e.g. used by our GitHub Action)
• cleaner separation of core and plugin layers - plugin-* packages can import from utils or models, but not from core
• the cli project might not then need multiple entry points (only bin)
• having separate cli and core is quite common in other CLI-first projects (e.g. jest or graphql-code-generator)

Nx graph

[BioPhoton]

As this refactoring is pretty involved in the whole code base i suggest the changes in the following order:

1. Prepare the codebase for the move - especially functions that will move should file in independent fils
   1.1. utils: refactor(models): add global options #76
   1.2. cli: refactor(cli): prepare for core package  #80
2. Create the new core package => it accidentally got pushed already. sry 💩
3. Move files from cli to utils and core
4. Move files from utils to core => feat(utils): move parts from utils into core #81
5. Cleanup refactoring => fix: refactor after core package intro #83

[matejchalk]

Closing since all the mentioned PRs have been merged.