fix(astro): Fix import path when using external init files with default path

[Lms24]

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

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.3 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.61 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.26 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.63 KB (+0.05% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.13 KB (0%)
@sentry/browser - Webpack (gzipped)	22.48 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	74.97 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	66.6 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.43 KB (+0.04% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.19 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	209.71 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	97.77 KB (+0.06% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	72.34 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	35.47 KB (+0.07% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69 KB (+0.02% 🔺)
@sentry/react - Webpack (gzipped)	22.52 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.62 KB (+0.02% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.75 KB (+0.04% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.11 KB (0%)

[Lms24]

Had to add a second test file here to avoid globally mocking the fs.existsSync implementation for the other tests which relied on its default implementation.

[HazAT]

🙏