While looking into #16263 I realised that we can drastically simplify the prop implementation now that deriveds are writable. Some of the code in red is pretty hard to understand, but deleting it causes no tests to fail. Just to be safe, I added a changeset so that this creates a new version that we can roll back from if it causes any issues.

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).

Tests and linting

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