[EngSys] Extract common functions from JS and TS code (.github and eng/tools)

[mikeharder]

Overview

1. Extract common functions from JS/TS code under .github and eng/tools
2. Place under .github/src
3. May not depend on any external packages OR github- or devops- specific functionality
4. Must support logging to console (dev inner-loop), GH logs, and devops logs. Auto-detecting the most likely environment.
5. Can be included by any other JS or TS code in specs repo (.github and eng/tools)

Suggested Functions

• Most of the functions from https://github.com/Azure/azure-rest-api-specs/blob/main/eng/tools/spec-gen-sdk-runner/src/utils.ts
• List of files changed in PR
  • Pre-defined filters for the list, like "resource-manager" and "exclude-examples"
  • const files = await getChangedFiles().filter(resourceManagerFilter).filter(excludeExamples)
• Consider using git diff-tree --no-commit-id --name-only -r ${{ github.sha }} instead of diff HEAD HEAD^.