chore: resolve ssr options

[antfu]

This PR adds a resolve function for SSR options to set the default. It allows plugin to know the target format:

const myPlugin = {
  configResolved(config) {
    const isESM = config.ssr?.format === 'esm'
  }
}

Otherwise, it was always undefined unless explicitly set, but the default is different in v2 and v3. This PR makes it easier for plugins to get the format without relying on checking Vite's version.