chore: remove $state.is from docs (#12927)

sites/svelte-5-preview/src/lib/autocomplete.js

@@ -118,7 +118,6 @@ const runes = [
 	{ snippet: '$bindable()', test: is_bindable },
 	{ snippet: '$effect.root(() => {\n\t${}\n})' },
 	{ snippet: '$state.snapshot(${})' },
-	{ snippet: '$state.is(${})' },
 	{ snippet: '$effect.tracking()' },
 	{ snippet: '$inspect(${});', test: is_statement }
 ];

sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md

@@ -108,26 +108,6 @@ To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snaps
 
 This is handy when you want to pass some state to an external library or API that doesn't expect a proxy, such as `structuredClone`.
 
-## `$state.is`
-
-Sometimes you might need to compare two values, one of which is a reactive `$state(...)` proxy. For this you can use `$state.is(a, b)`:
-
-```svelte
-<script>
-	let foo = $state({});
-	let bar = {};
-
-	foo.bar = bar;
-
-	console.log(foo.bar === bar); // false — `foo.bar` is a reactive proxy
-	console.log($state.is(foo.bar, bar)); // true
-</script>
-```
-
-This is handy when you might want to check if the object exists within a deeply reactive object/array.
-
-> `$state.is` uses `Object.is` to check if two values are the same value.
-
 ## `$derived`
 
 Derived state is declared with the `$derived` rune: