feat: generate CSS exports when css.splitting is disabled

[jinghaihan]

Description

Automatically generates CSS exports in package.json when css.splitting is disabled. This enables component libraries to bundle CSS into a single file and expose it via package exports for users to import.

Additional context

generateExports now accepts a single options object (containing exports, logger, css, outDir) instead of separate ExportsOptions and logger parameters. This change enables future extensions where exports generation depends on multiple configuration options.

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commit: 35b0ff2

[sxzz]

exports need to be determined based on the chunk, not just the CSS configuration and outDir.

The filename and outDir could be overridden by plugins/outputOptions.