Add programmatic interface to create-svelte

[AnatoleLucet]

This pr adds a programmatic interface to create-svelte so it can be called using a single function call.

Closes #3421

Not sure if the package should be bundled, since it is now exporting an entry point. Let me know if it needs to.

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
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• Ideally, include a test that fails without this PR but passes with it.

Tests

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Changesets

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[changeset-bot]

🦋 Changeset detected

Latest commit: 61a5f07

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

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[Rich-Harris]

excellent, thank you!