Add validate command

[amis92]

wham validate should read files in "safe" mode and check for the most basic problems.

This can include:

• migrating to newest format (should be optional)
• validating schema (should be basic, but may require migrating or creating schemas for previous versions - back to 2.00?)
• printing formatted, parseable output with line addresses to support using it in automated builds like GH Actions, Az Pipelines
• running other analyzers that might be configured via project settings (somehow)

Basic checks:

• unique file IDs across repo
• unique entry IDs in file
• schema-validness

[amis92]

Also allow an option like --resave-fixed to help quickly fix simple issues (like migrations).