Add spell checking to the plugin command line tool

See https://github.com/flutter/flutter-intellij/pull/1887 for an example of why this would be useful and a hint on how to implement it.

It could be part of `plugin lint`, for example.

Start by reviewing the linked pull request for the implementation hint and then locate the plugin lint command-line entry point. Define the spell-checking scope and make plugin lint report spelling problems consistently; verify the behavior through the command-line checks available in the project.

Written by the indexing model from the issue text.