feat: integrate env variable loading into createRsbuild options

[chenjiahan]

Summary

Although Rsbuild's JavaScript API already provides the loadEnv method, combining loadEnv with createRsbuild is still somewhat cumbersome.

This PR introduces a new loadEnv option for createRsbuild to simplify the logic of env loading.

const rsbuild = createRsbuild({ loadEnv: true });

This is equivalent to:

const envs = loadEnv();

const config = {
  // some options
};

config.source.define = {
  ...envs.publicVars,
  ...config.source.define,
};

// watch the env files
config.dev.watchFiles = [
  ...(config.dev.watchFiles ? castArray(config.dev.watchFiles) : []),
  {
    paths: envs.filePaths,
    type: 'reload-server',
  },
];

// add env files to build dependencies, so that the build cache
// can be invalidated when the env files are changed.
if (config.performance?.buildCache && envs.filePaths.length > 0) {
  const { buildCache } = config.performance;
  if (buildCache === true) {
    config.performance.buildCache = {
      buildDependencies: envs.filePaths,
    };
  } else {
    buildCache.buildDependencies ||= [];
    buildCache.buildDependencies.push(...envs.filePaths);
  }
}

const rsbuild = createRsbuild({
  rsbuildConfig: config,
});

rsbuild.onCloseBuild(envs.cleanup);
rsbuild.onCloseDevServer(envs.cleanup);

Checklist

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

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