feat: treat tag with . as a component, even if lowercase

[Rich-Harris]

Alternative to #12752. This PR aligns Svelte with other frameworks like Solid and React, by treating <foo.bar> as a component tag rather than an element tag, since it is an invalid element name.

There is a lingering question over whether this would be problematic for Svelte Native, but I'm confident that another solution could be found there.

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• 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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• 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: 519957a

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

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[trueadm]

Makes sense.

[Conduitry]

I think this change makes sense - but if we do this, we should include it in the list of breaking changes, since it also applies in non-runes mode.