Add .ts files in create-svelte/shared to both +typescript and -typescript (as .js files)

[Rich-Harris]

Alternative to #4365. With this PR, instead of needing to maintain parallel test/test.ts and test/test.js files (or whatever other stuff gets added in future), we can just have a .ts file in a shared directory that doesn't specify typescript, and it will get added to both +typescript and -typescript variants, appropriately converted in the latter case.

fixes #4341. closes #4365

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

[changeset-bot]

🦋 Changeset detected

Latest commit: ebe5af8

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

This PR includes changesets to release 1 package

Name	Type
create-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

[benmccann]

I don't know if there's any sort of test we should have for this?

[Rich-Harris]

re tests — maybe, though I kinda feel like that applies to the package as a whole. one day