fix(plugin-svelte): transpile the Svelte package to downgrade the syntax

[chenjiahan]

Summary

Transpile the Svelte package to downgrade the syntax. The package contains ES2020 syntax, such as optional chaining.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR modifies the Svelte plugin to transpile the Svelte package, ensuring that newer syntax (e.g., optional chaining) is downgraded. Key changes include:

• Importing EnvironmentConfig from @rsbuild/core.
• Adding an environment configuration hook via api.modifyEnvironmentConfig.
• Integrating the transpilation logic conditioned on the Svelte version.

Comments suppressed due to low confidence (1)

packages/plugin-svelte/src/index.ts:81

• [nitpick] Ensure that the regex pattern correctly captures all intended Svelte package paths across different environments. It might be useful to note any assumptions about path separators for future reference.

include: [/node_modules[\/]svelte[\/]/],