feat: update `watch` - Githubissues

abdel-17 commented 1 month ago

This PR changes two things about the watch helper.

Remove the once: boolean option in favor of a separate watchOnce function. The default behavior for watch is to run immediately, unlike in Vue where it runs lazily when a dependency changes, so having once: true behave just like onMount by default makes no sense. As an added bonus, watch is now much simpler internally.
To watch for changes to multiple values, you pass an array of getters. Previously, you would pass a getter to an array of state values, but this is kind of ambiguous. What if I want to watch for changes to a single state that happens to be an array?
```
let items = $state([]);
watch(() => items, (items, previousItems) => {
// `previousItems` is an empty array, but it should be `undefined`!
});
```