feat(rsc): support UserConfig.rsc: RscPluginOptions

[hi-ogawa]

Description

• Closes Support overriding RscPluginOptions via user config #806
• Closes Add empty builder.buildApp to workaround @cloudflare/vite-plugin build conflict #812

For example, this allows other plugin to control RscPluginOptions via its own config hook:

import { defineConfig } from "vite";
import rsc from "@vitejs/plugin-rsc";

export default defineConfig({
  plugins: [
    {
       name: 'rsc-ssg',
       config(_config, env) {
          return {
             appType: env.isPreview ? "mpa" : undefined,
             rsc: {
               serverHandler: env.isPreview ? false : undefined,
               ...
             }
          }
       }
    },
    rsc({
       // ... instead of config here ...
    }),
  ],
})

[Copilot]

Pull Request Overview

This PR adds support for configuring RSC plugin options through the top-level rsc property in Vite's user config, allowing other plugins to set options via the config hook.

• Adds support for UserConfig.rsc: RscPluginOptions to allow configuration through Vite's config system
• Refactors buildApp hook handling to use object format with conditional execution based on useBuildAppHook option
• Updates example configurations to demonstrate the new configuration approach

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

File	Description
packages/plugin-rsc/src/plugin.ts	Implements config merging for config.rsc and refactors buildApp hook handling
packages/plugin-rsc/examples/starter-cf-single/vite.config.ts	Demonstrates new rsc config property usage
packages/plugin-rsc/examples/ssg/vite.config.ts	Migrates from function-based to object-based config and uses new rsc property
packages/plugin-rsc/README.md	Documents the new configuration option

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