Dry out config

[Rich-Harris]

Simplifies the config validation without introducing new dependencies (follow-up to #2366 (comment))

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[bluwy]

Some things I found while refactoring in #2366:

1. target, adapter, host and hostHeader are nullable, but not reflected in Config and ValidatedConfig types.
   

   kit/packages/kit/types/config.d.ts

   Line 32 in 90874c7

   export interface Config {

2. Maybe we want to validate/coerce this together as well:
   

   kit/packages/kit/src/core/config/index.js

   Lines 39 to 46 in 90874c7

   	validated.kit.files.assets = path.resolve(cwd, validated.kit.files.assets);
   	validated.kit.files.hooks = path.resolve(cwd, validated.kit.files.hooks);
   	validated.kit.files.lib = path.resolve(cwd, validated.kit.files.lib);
   	validated.kit.files.routes = path.resolve(cwd, validated.kit.files.routes);
   	validated.kit.files.serviceWorker = path.resolve(cwd, validated.kit.files.serviceWorker);
   	validated.kit.files.template = path.resolve(cwd, validated.kit.files.template);
   	validate_template(cwd, validated);

[Rich-Harris]

Ah yeah, that's mildly annoying about the types, though if I amend Config to have string | null etc it causes more problems, so I'm okay glossing over that for the time being.

I considered making the homegrown validation logic accept options so that we could do the resolution in the validator, but when I implemented it the extra complexity outweighed those six extra lines, so I made the pragmatic choice. Can always revisit that in future if we end up with more options that need to be amended post-validation to account for things like cwd.