Add tab --check, a config validation utility

[austinjones]

Add a validation command that checks tab.yml for errors. There are many misconfigurations that aren't serious enough to break tab selection.

Recurse the current scope, discover all configs, and lint them:

• Check for non-parsable configs
• Check for incorrect config types (e.g. a repo link that contains a workspace config)
• Check for invalid tab names (validated by the command-line parser in tab/src/cli.rs)
• Warn on links with no config (e.g. a repo link with a valid path, but no config)

If a config parse error occurs during tab startup, suggest that the user use tab --check to lint their config files.

[austinjones]

Resolved by #207 and #208