[heft-jest] (BREAKING CHANGE) Use slash-normalized relative paths for asset URLs

[dmichon-msft]

Summary

Updates the jest-string-mock-transformer that is the default transformation for static assets to emit slash-normalized paths relative to config.rootDir, rather than absolute paths. This ensures that the result of the transform can be safely used in snapshot tests, and will continue to work if the path is passed directly into file system APIs, because the fs API resolves relative paths to the current working directory.

Details

The motivation for this change is that the current implementation of the transformer is unsafe to use in snapshot tests, since it returns platform-specific absolute paths.

Example:

src/image.test.ts

import myImage from './my-image.png';

// Path will use slashes and be relative to the current working directory
// It will be the relative path to the *output* file in the CommonJS output folder.
expect(myImage).toBe('lib-commonjs/my-image.png');

How it was tested

Added a build test case.

Impacted documentation

Docs on the default transformer.