fix: resolve scoped npm packages and warn on missing deps

[uriva]

Summary

Fixes two bugs in prefixPlugin.ts when resolving npm: specifiers. Closes #40.

Bug 1: Scoped package names parsed incorrectly

deno info --json returns npmPackage identifiers like:

• Unscoped: preact@10.25.4
• Scoped: @preact/signals@2.8.1_preact@10.25.4

The existing code uses indexOf("@") to find the version separator. For scoped packages starting with @, this returns 0, so slice(0, 0) produces an empty string "". The fix finds the @ after the / in scoped package names.

Bug 2: Silent externalization on resolution failure

When this.resolve(packageName) fails (package not in node_modules), the old code returned the bare package name as a string. Vite interpreted this as an external module and silently externalized it — the error only appeared at runtime in the browser as:

Module "xyz" has been externalized for browser compatibility

Now the plugin emits a warning with actionable guidance and returns undefined, allowing Vite to report the unresolved import at build time instead.

Why packages might not be in node_modules

This commonly happens when an npm: specifier comes from a JSR package. Deno resolves JSR transitive npm dependencies through its own module graph, but they don't get installed into the project's node_modules/. The warning tells users to add the package to their import map or package.json.

Tests

• Added extractPackageName unit tests covering unscoped, scoped, scoped with peer dep suffixes, and versionless package identifiers
• Added integration test for inline npm:@preact/signals (scoped package)
• All 17 tests pass (11 existing + 6 new)

[uriva]

Note: the lint-and-format check is failing, but this is a pre-existing failure on main. The no-import-prefix and no-unversioned-import rules flag all the inline test fixtures (inlineNpm.ts, inlineJsr.ts, inlineHttp.ts, etc.) which intentionally use inline specifiers since that's what the plugin resolves. My new inlineScopedNpm.ts follows the same pattern.

This PR fixes #40.

[uriva]

@marvinhagemeister could you ptal?