vue: add jest testing hints on imports and add exports to package.json

[lucas-koehler]

• Add documentation on import order for jest (or other) unit tests using CJS transformations
• Add explicit exports property to vue's package.json to facilitate tools finding the correct bundle

Closes #2250

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[coveralls]

coverage: 82.957%. remained the same — lk/2250-vue-test-import-order-doc into master

[lucas-koehler]

The "exports" seems to have caused test issues on windows and mac 😅

Reason:
The exports field is strict and relies on accurate realpath() resolution to determine package identity and resolve relative entry points. In a pnpm workspace monorepo, where packages are symlinked (or junction-linked on Windows), this resolution has subtle cross-platform differences that can cause the exports map to either fail entirely or resolve to the wrong bundle format — while the more lenient main/module fallback works reliably on all platforms.

This is a known category of issue in the Node.js ecosystem — exports + workspace symlinks + cross-platform is a notorious combination. Removing the exports field was the pragmatic fix.