test: support testing on Windows

[jason-ha]

• package.json_s: use compatible quoting
• use posix paths in createTestPackage
• convert \r\n to \n before comparing strings
  • correct naming of snapshot strings; deduplicate write logic
• resolve file relative paths with fileURLToPath
• add Windows to CI test matrix

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[andrewbranch]

This is great, thank you so much! Would you be up for adding a Windows runner to the CI workflow before merging?

[jason-ha]

This is great, thank you so much! Would you be up for adding a Windows runner to the CI workflow before merging?

I don't know what that is but I Will take a look. Can add another PR after if I don't figure it out straightaway.

[andrewbranch]

This example shows something like what I have in mind: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/running-variations-of-jobs-in-a-workflow#using-a-matrix-strategy