Migrate to sfdx plugin architecture

Changelog

Move to the sfdx plugin architecture!
sfdx plugins:install sfdx-git-delta
sgd command line is still supported

Added

• sfdx sgd:source:delta command via sfdx plugin
• v50.0 API

Changed

• -V parameter of sgd command is handled by sfdx plugins command
• Migrate from npm to yarn command line for development tooling
• Migrate from travis-ci to gh-actions
• -r parameter default value is now '.' instead of 'repo'

Deprecated

• warnings output attribut is not null anymore when no warnings are encountered. It is empty array.