Improvements to build and test processes

• Uses shx to run setup scripts so the package can be developed on non-Unix-like OSes such as Windows (Note: preview-npm-files is not Windows-compatible because shx does not support tar)
• Moves unit test files and jest config to separate tests file
• Adds ts-jest as a devDependency to run unit tests. Simplifies previous process of transpiling to a temporary directory.
• Removes duplicate or unused scripts in package.json
• Renames NPM deploy GitHub action to cd.yml
• CI runs only on PRs into main
• CD runs on merge/push into main