Migrate Azure/avocado to this repo

[mikeharder]

• Code under src, test, and cmd mostly copied from https://github.com/Azure/avocado
• Minor changes needed for formatting, folder structure, etc
• Aligns with existing tools in repo
• Upgraded dependency on yargs from 15 to 18
• Should be no observable change to PR checks or local usage
• Includes one bug fix, to prevent crashing on invalid readme.md (790ae88)
  • Discovered by workflow "Avocado - Analyze Code" when inspecting the test files in this PR

To Do

• index.ts:getInputFilesFromReadme() - return error "NOT_AUTOREST_MARKDOWN" instead of swallowing exception

[github-actions]

Next Steps to Merge

Next steps that must be taken to merge this PR:

• ❌ The required check named Swagger PrettierCheck has failed. Refer to the check in the PR's 'Checks' tab for details on how to fix it and consult the aka.ms/ci-fix guide

Comment generated by summarize-checks workflow run.