add paths: { base, assets } and appDir config

[Rich-Harris]

Adds the paths option from #229 (comment)

[Rich-Harris]

The design as things stand:

// svelte.config.js
module.exports = {
  kit: {
    paths: {
      base: '',
      assets: '',
    },
    appDir: '_app'
  }
};

paths.base is a root-relative pathname — if it is equal to /foo, then src/routes/bar.svelte equates to /foo/bar. paths.assets can be relative (in which case resolved against base, root-relative or absolute. They can be imported inside the app from $app/paths:

import { base, assets } from '$app/paths';

appDir is the directory that hashed JS/CSS/whatever chunks are written to. It must be a subdirectory of assets. It can't be imported from anywhere because there's no reason you'd want to do that.

---

This does introduce some new wrinkles. It doesn't make sense for adapter-static, for example, to put pre-rendered pages and assets in the same place if they're potentially on entirely different domains. But since in many cases that's exactly what you want, it needs to be configurable, so it probably needs to look something like this:

// svelte.config.js
module.exports = {
  kit: {
    adapter: ['@sveltejs/adapter-static', {
      // output everything to the same directory...
      pages: 'build',
      assets: 'build'
    }],
    paths: {
      // ...since there's no funny business with different domains:
      base: '',
      assets: '',
    }
  }
};

// svelte.config.js
module.exports = {
  kit: {
    adapter: ['@sveltejs/adapter-static', {
      // output assets to a separate directory...
      pages: 'build/pages',
      assets: 'build/assets'
    }],
    paths: {
      // ...so we can upload them to somewhere other than our website
      base: '',
      assets: 'https://cdn.example.com/buckets/xyz123',
    }
  }
};

Other adapters will likely have to accommodate these options somehow. For example what is adapter-node's responsibility viz static assets if paths.assets is an external URL?