fix: follow spec for customElement option

[wackbyte]

Fixes #13244. Follows the WHATWG spec when validating the customElement option in <svelte:options>.

This is technically a breaking change because the validator would no longer accept uppercase letters in customElement; however, any code that did do this is invalid and would error immediately at runtime anyway, so I argue that it isn't a breaking change.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8cc5dbd

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

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svelte	Patch

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[Rich-Harris]

thank you!