[feat] more specific types for kit.prerender.entries config

[HarryAllen1]

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 pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

This PR adds stricter typings for config.kit.prerender.entries.
Before, editors won't flag the following as incorrect (but the compiler will):

/** @type {import('@sveltejs/kit').Config} */
const config = {
	kit: {
		prerender: {
			entries: ['afdasd'],
		},
	},
};

With this PR, the type of entries is stricter, making it easier to debug before building.

[changeset-bot]

🦋 Changeset detected

Latest commit: 62b76f6

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

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]

Thank you!