feat: add |nonself event modifier for non-interactive elements with event listeners

[oscarhermoso]

Fixes #8964

Currently, a few a11y warnings fire for the false positive scenario of non-interactive elements listening to bubbled events.

This PR adds a new |nonself event modifier, that ignores events emmited by the listener's own element (eg. only listens to bubbling events)

This improves the following:

• Avoids inexperienced developers blindly following warnings and adding a tabindex to non-interactive divs
• Complements the existing self modifier, reducing the amount of boiler-plate code to ensure an event has bubbled from a child
• Gives a pathway to resolve false positive warnings safely, rather than ignoring them

Before submitting the PR, please make sure you do the following

• 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
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• 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: 6773057

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

This PR includes changesets to release 1 package

Name	Type
svelte	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[oscarhermoso]

Raised an accompanying PR in sveltejs/language-tools:

sveltejs/language-tools#2110

[oscarhermoso]

Going to close this considering that Svelte 5 won't have event modifiers