refactor(core/loadConfig): loadConfig({ cwd, path})

[SoonIter]

Summary

1. refactor loadConfig

// before
export async function loadConfig(
  customConfig?: string,
  envMode?: string,
): Promise<RslibConfig> 

// after  https://github.com/web-infra-dev/rsbuild/blob/080a0d856f6743fe1e9570638448a3d629f91dea/packages/core/src/config.ts#L347
export async function loadConfig({
  cwd = process.cwd(),
  path,
  envMode,
}: {
  cwd?: string;
  path?: string;
  envMode?: string;
}): Promise<RslibConfig> 

2. remove the first arg of generateEsmConfig