Feat: support externals field in assets-manifest

[ClarkXia]

This pull request introduces changes to support an externals field in the assets manifest, enabling external scripts to be dynamically injected into the application. The updates span multiple files, including the @ice/plugin-externals and @ice/runtime packages, and involve both functionality enhancements and type definitions.

[Copilot]

Pull Request Overview

This PR adds support for an externals array in the assets manifest so that external scripts can be injected at runtime.

• Introduce an optional externals field in the manifest type
• Emit and populate the externals array in the Webpack plugin
• Render external scripts in the Document component

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/runtime/src/types.ts	Add externals?: string[] to AssetsManifest
packages/runtime/src/Document.tsx	Map over assetsManifest.externals to inject scripts
packages/plugin-externals/src/webpack-plugin.ts	Initialize and unshift externals into the JSON output
.changeset/fluffy-items-argue.md	Record the new feature in the changeset

Comments suppressed due to low confidence (2)

packages/runtime/src/types.ts:208

• Consider adding a JSDoc comment explaining the purpose of the externals field (e.g., expected value format and how it's used).

  externals?: string[];

packages/plugin-externals/src/webpack-plugin.ts:30

• Add unit tests for InjectExternalScriptsWebpackPlugin to verify that the externals field is created and populated correctly in the emitted manifest.

            json.externals ||= [];