feat: add packageJson setting

[dummdidumm]

This adds a new config option packageJson: (originalPackageJson, generatedPackageJson) => packageJsonYouWant | undefined

• closes Allow package.json customisation when using svelte-package #7258
• closes @sveltejs/package Allow disabling the default behavior of copying package.json to the output directory when packaging a Svelte library #6824
• closes Making package handle more complex use cases. #4828
• part of Change "svelte" path in package.json in root during dev and in package dir after npm run package #2242
• better message for Remove the need for an index file with npm run package #2884

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

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• 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. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[changeset-bot]

🦋 Changeset detected

Latest commit: 45bef9e

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/package	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Rich-Harris]

I'll close this since we settled on a different approach (#8825)