feat!: add prestart and poststart as adapter options

[devjume]

This PR will add prestart and postart as adapter options and pass them them to generated package.json

Example use cases:

• When using bun:sqlite I want to save sqlite database on it's own folder. With prestart script I can run mkdir ./database/ automatically.
• with prestart user can run migration scripts before starting production version of sveltekit app.

npm Docs - prestart & poststart

Example how to use:

// svelte.config.js

import adapter from "svelte-adapter-bun";
const config = {
  kit: {
    adapter: adapter({
      prestart: "mkdir ./database",
      poststart: "echo 'poststart'",
    })
  }
};

// generated package.json in build folder

"scripts": {
  "prestart": "mkdir ./database",
  "poststart": "echo 'poststart'",
  "start": "bun ./index.js"
},