This PR fixes a pretty obvious bug in the Astro SDK where specifying an external sentry.(client|server).config.js in the default location file would cause a build error. Looks like this slipped through at some point and we didn't test it properly.

Previously, we checked for a supported file extension and instead of adding an import statement for the path to the file, we'd only add the file extension. So

import 'mjs';
// instead of 
import 'path/to/sentry.client.config.mjs'

This PR fixes the filename lookup function and adds a test to cover regressions.

cc @HazAT