fix: narrow defaultChecked to boolean

[hyunbinseo]

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
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

checked?: boolean | undefined | null;
// ...
// match the existing checked type
defaultChecked?: boolean | undefined | null;
defaultchecked?: boolean | undefined | null;

Was implemented here: 6d0b324

• feat: default values for form elements #14289

Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: c03fa51

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

This PR includes changesets to release 1 package

Name	Type
svelte	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

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16009-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16009

[Rich-Harris]

thank you!