Remove Sketchbook concept, introduce User data folder

[masci]

Rationale

After some QA iterations we decided to fade out the Sketchbook concept from the CLI. Please note the Sketchbook will continue existing on the IDEs, just it will be implemented at application level, not at the CLI level.

arduino-cli sketch new and other commands already stopped relying on the Sketchbook, assuming the current path will be used when no destination path is provided but the "Sketchbook" keyword continued to be present in some places in the CLI, most notably the configuration file, causing confusion among users.

Implications

The Sketchbook isn't only used to collect sketch files, it also contains custom libraries and custom hardware under the paths <sketchbook_path>/libraries and <sketchbook_path>/hardware respectively - this functionality will be preserved by introducing a new concept at the CLI level: the User folder.

This PR removes the Sketchbook dir from the settings and introduces a new concept, the "User" folder. User folder will have the same layout as the existing "Data" folder and will contain "custom stuff" like libraries and hardware. Users can keep using it to store sketch files, just the CLI won't care nor assume anything about those.

Backward compatibility

In order to keep backward compatibility, the environment var ARDUINO_SKETCHBOOK_DIR will keep existing and point to the User folder. The old sketchbook_path value will be removed from the settings instead and user will use this new configuration value in arduino-cli.yaml:

directories:
  data: /home/massi/.arduino15
  user: /home/massi/Arduino

gRPC interface

At this moment the gRPC won't be touched and will keep showing references to the Sketchbook. That part will be refactored to improve the way gRPC clients can pass configuration values without having to rely on a config file.

[cmaglie]

Everything looks good to me except:

The old sketchbook_path value will be removed from the settings instead and user will use this new configuration value in arduino-cli.yaml

I'd keep this option so users that have an old config file will not suffer breaking changes.
I'm ok to remove it from config dump so new configurations will use the new values.

[rsora]

@cmaglie keeping the sketchbook_path options unfortunately will continue to create issues as #503 in which users are not able to figure out why the sketchbook concept does not work properly in the CLI.

I suggest to go ahead and merge, we will explain precisely in the changelog the effects of this change and how to migrate the arduino-cli.yaml to be used under the next release.

[masci]

@pirropirro FYI we're merging this