implement config.kit.alias

[paperclover]

This PR implements config.kit.alias, as the proposed solution in #4734 describes.

Example usage:

const config = {
  kit: {
    alias: {
      $utils: 'src/utils'
    }
  }
};

Notes:

• I would like some pointers/assistance with adding the new config property, I may have messed something up.
• New tests need to be written and some existing ones need to be modified. Before I go on with writing tests, I'd like feedback to the code itself.
• We should consider deprecating/removing config.kit.files.lib and having it be config.kit.alias.$lib, but $lib is special for packaging and other situations.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

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• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: ab2b77a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[benmccann]

looks pretty good to me. main things I see are the failing tests and need to update the documentation

[paperclover]

looks pretty good to me. main things I see are the failing tests and need to update the documentation

i just merged in your change requests, and i'll get to tests and docs today.

[paperclover]

some notes/questions

1. is this doc page alright? link
2. should there be any extra validation on alias key and value? i havent tested much malformed input on them (such as an absolute path).
3. do we need more tests besides fixing the broken ones? im not 100% familiar with the codebase so if you want tests for making sure aliases do their job, id need some pointers.
4. should the $lib alias be moved into this new config? i feel like it should but i also don't want to break anything, or put too much in one commit.

[Rich-Harris]

thank you!