feat: make internal Svelte Custom Element treeshakable

[minht11]

When inspecting my own application I noticed that despite not using any custom elements Svelte still includes code for it. This PR marks Svelte Custom Element with pure annotation so when it is not used it can safely removed.

I was not able to make it work with conditional assigment so I moved class definition into function, in my testing this reduces size of minified code by ~2kb.

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 802afc6

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

[dummdidumm]

Thank you! Which bundler are you using? We have a treeshaking check using Rollup in our test suite which didn't seem to have caught this.

[minht11]

Rolldown which possibly is not yet smart enough to treeshake this. Will test it with Rollup later.

[minht11]

Checked with Rollup and seems like Rolldown issue indeed, likely rolldown/rolldown#4258.