feat: enhance api.transform typing with conditional types

[chenjiahan]

Summary

Previously, the code property of api.transform was always typed as string | Buffer, requiring type checking even when the raw mode was known at compile time:

api.transform({ test: /\.node$/, raw: true }, ({ code }) => {
  if (Buffer.isBuffer(code)) {
    // 
  }
});

This PR implemented conditional typing that provides precise types based on the raw option:

// When raw: true, code is definitively Buffer
api.transform({ test: /\.node$/, raw: true }, ({ code }) => {
  // code is typed as Buffer
  emitFile(name, code);
});

Related Links

close #6017

Checklist

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

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[Copilot]

Pull Request Overview

This PR adds type-safe support for the raw option in transform hooks, ensuring that when raw: true is specified in a transform descriptor, the handler receives a Buffer instead of a string for the code property.

Key changes:

• Made TransformContext and TransformHandler generic with a Raw type parameter
• Updated TransformHook to infer the raw property from the descriptor and provide correct typing to the handler
• Updated the transformer storage to accommodate handlers with different raw configurations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/core/src/types/plugin.ts	Added generic type parameters to TransformContext, TransformHandler, and TransformHook to support type-safe raw Buffer handling
packages/core/src/initPlugins.ts	Updated transformer record type to accept handlers with either raw or string code types

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