Add possibility to update Dart SDK version of sidekickDartRuntime

[Giuspepe]

Currently the sidekickDartRuntime's Dart SDK version can only be updated by increasing the minimum version constraint in the sidekick CLI's pubspec.yaml and running the CLI's entrypoint. This is not very intuitive and unnecessarily coupled. So we should add another way to update the sidekickDartRuntime's Dart SDK version.

We could add it to the sidekick update command, e.g. like this: sidekick update <{cli (default), dart}> [{<version>, latest}]

sidekick update cli or sidekick update to update the sidekick CLI (current implementation of the command)

sidekick update dart to update sidekickDartRuntime (new)

[passsy]

I'd make it a separate command <cli> sidekick upgrade-embedded-sdk

[passsy]

Updating the embedded dart sdk is now possible with <cli> sidekick update.

@Giuspepe Please update the error message

sidekick/sidekick_core/lib/src/commands/plugins/install_plugin_command.dart

Line 255 in 914bde5

// TODO update instructions when https://github.com/phntmxyz/sidekick/issues/149 is resolved

[Giuspepe]

Updated the error message in #176

I guess we can close this issue then for now.

In the future it would still be nice to have a separate upgrade-embedded-sdk command and to save the embedded sdk version in the pubspec in the sidekick block instead of parsing it from environment: sdk: ..., i.e.

sidekick:
  cli_version: 0.13.1
  embedded_sdk_version: x.y.z