Support external sourcemaps

[dschulten]

Debugging the Svelte unit test is difficult because external sourcemaps seem not to be recognized or Cypress does not serve them to the browser.

Workaround: use internal sourcemap using webpack devtool option cheap-module-eval-source-map

Conceptually it seems external map files cannot be served easily, the app sources are requested with a single request like http://localhost:57301/__cypress/tests?p=cypress\integration\spec.js-619 and if that javascript points to a relative sourcemap URL at spec.map.js, the sourcemap apparently cannot be found.

This might be a cypress issue, but only for unit-test helpers like this one. Hence I add this issue here.