feat: Build and export library in CommonJS format

[eliot-akira]

This pull request adds a build script to compile the library to CommonJS format using a minimal Rollup setup; and an exports property in package.json (here) so the published NPM package can be consumed in both CJS and ESM formats.

This is related to:

• fix: Use fork of path-browserify converted to ES Module with individually exported methods isomorphic-git#2035

The build/test pipeline in the above PR currently fails due to the step build.docs using CommonJS require.

nps is executing `build.docs` : node ./__tests__/__helpers__/generate-docs.cjs
...
Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: /home/vsts/work/1/s/node_modules/@isomorphic-git/path-browserify/index.js
require() of ES modules is not supported.

Using the GitHub URL to add @isomorphic-git/path-browserify as a dependency will only work if the repo includes the built index.cjs.

---

Next steps:

• Remove build artifact index.cjs and exclude in .gitignore
• Merge this PR after any other suggested changes
• doc: Update README.md to reflect current API #2
• Publish the next version of @isomorphic-git/path-browserify to NPM
• Update the PR isomorphic-git/isomorphic-git#2035 to use the published NPM package instead of GitHub URL. Confirm the build/test pipeline succeeds.

@jcubic

[jcubic]

There are some errors from npm install in GitHub workflow.

[eliot-akira]

It looks like package-lock.json went out of sync somehow. npm install didn't fix it, so I removed my local node_modules and package-lock.json and regenerated it.

[jcubic]

Is it ready to publish to NPM?

[eliot-akira]

Yes, I think after the README is updated (#2) it's ready to be published.

I did a manual test locally using npm link in the isomorphic-git repo (isomorphic-git/isomorphic-git#2035), and as far as I can tell, all build and test steps are able to complete successfully using this fork (ESM and CommonJS).