css: resolve bare module specifiers in @import to node_modules

[vijaygovindaraja]

Fixes microsoft/vscode#295074

Summary

CSS @import statements with bare module specifiers (e.g. @import "foo/bar.css") currently do not resolve to node_modules, even though SCSS and LESS already support this. This PR fixes two issues:

1. Enable resolveModuleReferences for CSS — the node_modules fallback path was disabled for CSS (false) while SCSS and LESS both had it enabled (true). This aligns CSS with the existing behavior.

2. Fix getModuleNameFromPath() for bare specifiers — the function returned an empty string for module names without a slash (e.g. "bootstrap"), causing resolution to silently fail. It now returns the full path as the module name.

This was originally attempted in microsoft/vscode#305900 at the getDocumentContext level, but @aeschli pointed out it belongs here in the language service.

Changes

File	Change
src/cssLanguageService.ts	Set resolveModuleReferences: true for getCSSLanguageService()
src/services/cssNavigation.ts	Return full path from getModuleNameFromPath() when no slash present
src/test/css/navigation.test.ts	Add test for bare module specifier resolution
test/linksTestFixtures/node_modules/foo/bar.css	Test fixture

Test plan

• All 709 existing tests pass
• New test: @import "foo/bar.css" resolves to node_modules/foo/bar.css
• Existing tilde-prefix resolution (~foo/hello.html) continues to work
• Local file resolution takes precedence over node_modules fallback

[vijaygovindaraja]

@aeschli

[vijaygovindaraja]

@aeschli moved here from microsoft/vscode#305900 per your feedback. All 709 tests passing.

[vijaygovindaraja]

Hey @aeschli @alexr00 — noticed this has two approvals, is there anything else needed before it can land?