feat: Transform relative "sources" paths in source map

[bengourley]

When compiling a directory with Babel, the paths in the sources array of each source map contains relative paths from the output directory to the original file, e.g. ../../src/lib/logger.js.

This change resolves that path relative to the projectRoot setting so that the paths look correct in the dashboard.

Before

After

Testing

Unit tests have been added for this.

You can test this out manually (as I did for the screenshots above) using the Babel example project.

Be sure to replace the API key in examples/babel/package.json and examples/babel/src/services/bugsnag.js before you begin.

cd examples/babel
npm run build
npm run upload-source-maps
npm start

Compare the results on this branch with the results on the directory-mode master branch (driectory-mode is now merged) .

[tomlongridge]

Reviewed - all good.