Add test for whitenoise serving to avoid regression

[thclark]

Feature request

The fix in #25 requires an edit to the source map line at the end of the bundled javascript for svelte-jsoneditor.

When upgrading in future, this is an area where it'd be very easy to create a regression, because it'd be my tendency to forget this happened and dump the new JS straight into that file.

We want reliable releases without regressing this bug.

Proposed Solution

Suggest that whitenoise is included in our development dependencies and a test case is introduced that overrides settings to use whitenoise to serve the JS.