Deprecate --fast, add an option for --validate

[belav]

CSharpier is mature enough that we no longer need to validate every time code is formatted.

• Deprecate the --fast option. By default csharpier will not validate the changes.
• Add a new option for --validate. This can be used to validate the changes csharpier makes
• Update doc to recommend that --validate is used for the initial formatting commit to a new repo.