chore: use custom event constructor instead of deprecated

[tronicboy1]

Problem

The custom_event function uses document.createEvent which is a deprecated method.

https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/initCustomEvent

Solution

use the constructor method, new CustomEvent(...), instead.

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Tests

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[tronicboy1]

Needed to add the CustomEvent constructor to the global context for tests.

[dummdidumm]

AFAIK this is kept that way to support old browsers which don't have this global yet. For Svelte 5 I would probably in favor of moving to the new event constructor, if it can be polyfilled.

[tronicboy1]

I can understand that. Polyfilling is a can of worms I don’t think you want to open.

We could add an if clause for browsers that don’t have the global CustomEvent.

It’s been 3 years since the discussion for this implementation happened; now 99% of browsers support CustomEvent. It could be time to reconsider.

[Conduitry]

It will be time to reconsider, when we are releasing a new major version of Svelte where we permit ourselves to make breaking changes. We can't just say "it's been several years, so we won't consider dropping support for these browsers a breaking change anymore".

[tronicboy1]

@Conduitry

It will be time to reconsider, when we are releasing a new major version of Svelte where we permit ourselves to make breaking changes. We can't just say "it's been several years, so we won't consider dropping support for these browsers a breaking change anymore".

@Conduitry 100% agree with you, it would be a breaking change making it a major version change and I understand that now.

My apologies for coming across a bit rash, it's my first time attempting to contribute here 😅.

If there are other plans in place to fix some of the deprecated uses feel free to close this PR.