fix: null and warnings for local handlers

[adiguba]

• Closes Better error message when using a non-function as an event handler #15411

Fix some problems with event-handlers declared as local variables :

• Delegated events ignore the null value.
• The event_handler_invalid is not printed for invalid handlers.

And some little fix for all event-handlers :

• The event_handler_invalid will print the correct name for capture event (ex: onclickcapture instead of onclick).
• The event_handler_invalid warning will not hide the TypeError "xxx is not a function"
• Do no use "hoisted function" when the function do no have hoisted_params.

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🦋 Changeset detected

Latest commit: 2671dc6

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

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15460-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15460

[Rich-Harris]

nice — made one small tweak, to prevent the deopt in the case of things like

let onclick = () => {...};

which are essentially equivalent to function declarations, taking advantage of #15359. There may be other places where we can take advantage of is_function