Improve react build performance

[tylers-username]

• Replace react-code-blocks with react-refractor. It uses the same underlying prism package for output, but with reduced out-of-the-box-provided languages.
• Improve tests to check gutter line number matches .upsun/config.yaml service lines

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Old

[github-actions]

Hello, and thank you for contributing to the Upsun demo project!

Setting up tests

Until all tests pass, a pull request will not be merged.

Build tests for combinations of Python and Node.js will happen automatically, and must pass in order to merge.
Another test will actually runthrough all steps of the demo on a new project, but requires a repository admin to add the runthrough label to the pull request.

Failing tests

If your PR is failing, see the Running tests section of the README for troubleshooting details.

Reviewing these changes locally:

1. Clone this repo

   git clone -b improvement/react-package-build-time git@github.com:platformsh/demo-project.git demo-test-improvement/react-package-build-time && cd demo-test-improvement/react-package-build-time

2. Create a project on Upsun:

   upsun project:create --title "Demo Project test (pr-441/merge)" --default-branch improvement/react-package-build-time

   This command will set the local repository remote automatically, but you will need to specify the organization and region.

3. Push to Upsun:

   upsun push