config.kit.paths.assets does not support absolute URLs without a scheme

Describe the bug

The documentation for paths.assets says it requires and "absolute path", but any value not specifying a scheme (e.g. https://, http://) will be rejected with the following error:

config.kit.paths.assets option must be an absolute path, if specified. See https://kit.svelte.dev/docs/configuration#paths

Any path starting with a / is a valid absolute path, so either the documentation needs updating or the validation check needs to be reworked, here:

https://github.com/sveltejs/kit/blob/f766a54d76c8df0389eb28984af94d7b2a172ce7/packages/kit/src/core/config/options.js#L169

My guess is the reason for implementing this feature was to host assets from a CDN which lead to the current validation. However, there are equally valid uses for this setting that would benefit from not having to specify the domain.

In my case, I'm using the static adapter (SPA) with the pages and assets settings defined. I want to serve all static assets from /static/* but the only way to get this to work with the %svelte.assets% variable is to hard code the domain. To avoid this I at least need to hard code the /static value for any of my own static content referenced in app.html. I haven't confirmed this, but I suppose this hard coding approach would not even work with with the built js files which also need access to the assets prefix.

Reproduction

termina

Logs

❯ npm run build
$ svelte-kit build
> config.kit.paths.assets option must be an absolute path, if specified. See https://kit.svelte.dev/docs/configuration#paths
    at eval (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:632:14)
    at eval (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:798:43)
    at eval (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:784:18)
    at eval (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:784:18)
    at eval (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:784:18)
    at validate_config (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:968:9)
    at load_config (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:945:20)
    at async eval (file:///home/projects/sveltejs-kit-template-default-7pbyt8/node_modules/@sveltejs/kit/dist/cli.js:1080:19)

System Info

System:
    OS: Linux 5.16 Arch Linux
    CPU: (24) x64 AMD Ryzen Threadripper 1920X 12-Core Processor
    Memory: 43.97 GB / 62.71 GB
    Container: Yes
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 17.4.0 - /usr/bin/node
    npm: 8.5.0 - /usr/bin/npm
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.17
    @sveltejs/adapter-static: ^1.0.0-next.28 => 1.0.0-next.28
    @sveltejs/kit: next => 1.0.0-next.278
    svelte: ^3.44.0 => 3.46.4

Severity

annoyance

Additional Information

No response

Start in packages/kit/src/core/config/options.js at the validation linked in the report, then reproduce the failure with the linked StackBlitz project and its static adapter configuration. Determine whether slash-prefixed asset paths such as /static should be accepted and whether the paths documentation at the linked URL matches the intended behavior; done means the reported configuration no longer fails unexpectedly or is documented accurately.