Add coverage comparison script

[douglaswinter]

A script that takes two dirs containing coverage summaries for each app and package, measured at two points (i.e. PR vs main) and fails if regression for any project is detected.

This script needs to be merged before a PR is created with a workflow action referencing it, as that PR itself will trigger the job and fail if the script is not found.

Fixes #26

[douglaswinter]

The reviewer may mimic what the upcoming workflow action will do by:

1. create new branch, let's say 'cov-test'
2. affect coverage (e.g. delete test file, or whatever case you want to cover)
3. commit
4. Run coverage: pnpm turbo coverage --filter=...[main] --force --no-daemon
5. Save report:
   mkdir pr-coverage
   find . -name coverage-summary.json | while read file; do
   project=$(dirname "$(dirname "$file")")
   project=$(basename "$project")
   cp "$file" "pr-coverage/$project.json"
   done
6. checkout main, run coverage: pnpm turbo coverage --filter=...[cov-test] --force --no-daemon
7. save report:
   mkdir base-coverage
   find . -name coverage-summary.json | while read file; do
   project=$(dirname "$(dirname "$file")")
   project=$(basename "$project")
   cp "$file" "base-coverage/$project.json"
   done
8. You'll have two dirs base-coverage an pr-coverage, each with a json file per project. Run the script: node scripts/compare-coverage.mjs pr-coverage base-coverage