Add config migration helper

[ecraig12345]

Add a beachball migrate command which checks for potential upgrade-related issues.

In theory the command could do codemods if needed in the future, but for now, it seems adequate to just warn about potential problems. (Also, both of the issues listed below are not guaranteed to be problems, so we shouldn't automatically change them anyway.)

• \ in any patterns: still valid as an escape but not as a path separator
• leading ! in exclude patterns: no longer required, but possibly still valid if someone wants to use it (not sure if it would work properly?)
• Warn about md-only changelog default change