fix(virtual): support default imports with Node16 and NodeNext

[Boulea7]

Rollup Plugin Name: @rollup/plugin-virtual

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers:

Fixes #2024

Description

With Node16 or NodeNext module resolution, an ESM consumer's documented default import is typed as a namespace and cannot be called:

import virtual from '@rollup/plugin-virtual';
virtual({ entry: 'export default 42' });

Route the import condition to an ESM declaration that reuses the existing function type. Existing runtime entries, CJS declarations, top-level types, and the default fallback are preserved.

The regression checks public package-name imports for ESM and CJS, plus a directory-entry fixture for legacy Node resolution. From packages/virtual, run pnpm test:ts.

Validation:

• pnpm build, pnpm test (7 tests), and pnpm test:ts pass.

• TypeScript 4.8.4 and 5.9.3 pass the Node16/NodeNext ESM and CJS checks and legacy resolution fixture.

• Public package-name ESM and CJS runtime imports pass against the rebuilt package.

• Root pnpm lint passes with 0 errors and 45 warnings in unchanged files in other packages; virtual has no warnings.

• Root pnpm fix:js passes with 0 errors and 39 warnings.

Draft pending maintainer approval for GitHub Actions.