fix: Prevent premature keydown interruptions during actions tutorial

[spacepumpkin]

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Closes #700

Bug:

• When following the actions tutorial to edit the actions.svelte.js file, key presses and focus are interrupted after adding use:trapFocus, likely due to the focusable()[0]?.focus(); within the $effect.

Test Steps:

• Go to tutorial here: https://svelte.dev/tutorial/svelte/actions
• Follow instructions from start to finish
• Expect not to have keypresses interrupted when editing the actions.svelte.js file

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[Rich-Harris]

Thanks. This is a clever approach but is ultimately just working around the bug, rather than fixing it, so I'll close it in favour of #1258

[spacepumpkin]

@Rich-Harris Works for me, thanks for fixing!