fix: show correct errors for invalid runes in .svelte.js files

[navorite]

fixes #12031

there is also a couple of other validation errors that I didn't add that happen inside .svelte files only e.g. Cannot use rune without parentheses but that would require checking member expressions as well

Svelte 5 rewrite

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

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.

Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 239040d

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

[Rich-Harris]

Thank you! The fix is simpler — we just need to move the existing validation logic into a place where it's used for everything. We don't want to do it every time we call get_rune, because that happens in a whole bunch of places when validation should only happen once.