[breaking] Implementation of sketch profiles

[cmaglie]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?
This PR implements Profiles.
In the long run, profiles will eventually replace and deprecate sketch.json.

Does this PR introduce a breaking change, and is titled accordingly?
This PR introduces breaking changes in the golang API and the cli behavior.

[per1234]

sketch-profiles.md should be added to the nav[] key in /mkdocs.yml so that the page is listed in the navigation panel of the documentation website. I recommend placing it after the "Sketch Specification" page:

arduino-cli/mkdocs.yml

Line 109 in 6883b39

- sketch-specification.md

[cmaglie]

sketch-profiles.md should be added to the nav[] key in /mkdocs.yml so that the page is listed in the navigation panel of the documentation website. I recommend placing it after the "Sketch Specification" page:

I've also renamed sketch-profiles.md in sketch-project-file.md, since the sketch.yaml will be surely extended in the future to support more features.

[per1234]

Thanks @cmaglie and to the other contributors for this valuable advancement for the Arduino project.

The failing link check for https://www.arduino.cc/en/Reference/APIStyleGuide is unrelated to the changes made in this PR. I am monitoring that and will take action if needed to get the correct redirect set up.

[cmaglie]

Just rebased on master to fix the dead link, we should be all-green in a couple of minutes 🥳