Set custom element data fails when using builtin custom elements.

[edoardocavazza]

Describe the bug

The helper set_custom_element_data only checks for autonomous custom elements but fails to check for builtin custom elements (is attribute) here, so it always set the value as attribute.

Reproduction

https://svelte.dev/playground/46f14880c73443318e26dd375ff3c0d2

Logs

System Info

REPL

Severity

annoyance