Support Svelte v4

[danawoodman]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [x] Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• [x] Ensure the tests and linter pass
• [x] Code coverage does not decrease (if any source code was changed)
• [x] Appropriate docs were updated (if necessary)

Fixes #269 🦕

[denuser]

Any chance we have it merged this week?