Fix browser compatibility for @xmcl/system by adding exports field

[Copilot]

This PR fixes browser compatibility issues with the @xmcl/system package when used with modern bundlers like Vite.

Problem

Users were getting compilation errors when trying to use @xmcl/system in browser environments:

[plugin:vite:resolve] Module "fs/promises" has been externalized for browser compatibility, imported by "node_modules/@xmcl/system/dist/index.mjs"
[plugin:vite:resolve] Module "path" has been externalized for browser compatibility, imported by "node_modules/@xmcl/system/dist/index.mjs"

RollupError: "stat" is not exported by "__vite-browser-external", imported by "node_modules/@xmcl/system/dist/index.mjs"

Root Cause

The package already had browser-compatible builds (index.browser.ts → dist/index.browser.mjs) that use JSZip instead of Node.js fs APIs. However, modern bundlers like Vite weren't using the browser build because the package.json lacked the modern exports field for conditional exports.

Without exports, bundlers fell back to the module field which pointed to the Node.js ESM build (dist/index.mjs) containing fs imports.

Solution

Added an exports field to packages/system/package.json with conditional exports:

"exports": {
  ".": {
    "browser": "./dist/index.browser.mjs",
    "import": "./dist/index.mjs", 
    "require": "./dist/index.js"
  }
}

This tells bundlers:

• Browser environments: Use ./dist/index.browser.mjs (JSZip-based, no Node.js APIs)
• Node.js ESM: Use ./dist/index.mjs (Node.js fs APIs)
• Node.js CommonJS: Use ./dist/index.js (Node.js fs APIs)

Testing

• ✅ All existing tests pass (24/24)
• ✅ Verified browser build contains no Node.js imports
• ✅ Verified resolution logic works correctly for different environments
• ✅ Created validation tests confirming the fix

Now when users import @xmcl/system in Vite or other browser bundlers, they'll automatically get the browser-compatible build instead of the Node.js version.

Fixes #307.

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