Filter out Svelte's unexpected class prop console warnings

Astro's hydration code passes a `class` prop to Svelte components, inducing Svelte to log a warning about an unknown prop. Preempting this by exporting a `class` prop from the Svelte component isn't a viable workaround since `class` is a reserved identifier in JS.

This PR implements the console-filtering workaround suggested by @HiDeoo in withastro#5665, borrowing the `useConsoleFilter` approach from the [preact integration](https://github.com/withastro/astro/blob/a1c0cbe604c9f91cdc421b5606aab574999eba01/packages/integrations/preact/src/server.ts#L72-L94).

It would probably be better to generalize console filtering so it could be shared across multiple integrations.

Ideally there would be a way to handle this in Svelte, but as was pointed out in the issue thread even they resort to [similar cringe-inducing hackery](https://github.com/sveltejs/kit/blob/master/packages/kit/src/runtime/client/client.js#L1974-L1996) in sveltekit.

packages/integrations/svelte/client.js

@@ -1,24 +1,33 @@
 const noop = () => {};
 
+let originalConsoleWarning;
+let consoleFilterRefs = 0;
+
 export default (target) => {
 	return (Component, props, slotted, { client }) => {
 		if (!target.hasAttribute('ssr')) return;
 		const slots = {};
 		for (const [key, value] of Object.entries(slotted)) {
 			slots[key] = createSlotDefinition(key, value);
 		}
+
 		try {
-			new Component({
-				target,
-				props: {
-					...props,
-					$$slots: slots,
-					$$scope: { ctx: [] },
-				},
-				hydrate: client !== 'only',
-				$$inline: true,
-			});
-		} catch (e) {}
+			useConsoleFilter();
+			try {
+				new Component({
+					target,
+					props: {
+						...props,
+						$$slots: slots,
+						$$scope: { ctx: [] },
+					},
+					hydrate: client !== 'only',
+					$$inline: true,
+				});
+			} catch (e) {}
+		} finally {
+			finishUsingConsoleFilter();
+		}
 	};
 };
 
@@ -51,3 +60,56 @@ function createSlotDefinition(key, children) {
 		noop,
 	];
 }
+
+/**
+ * Reduces console noise by filtering known non-problematic warnings.
+ *
+ * Performs reference counting to allow parallel usage from async code.
+ *
+ * To stop filtering, please ensure that there always is a matching call
+ * to `finishUsingConsoleFilter` afterwards.
+ */
+function useConsoleFilter() {
+	consoleFilterRefs++;
+
+	if (!originalConsoleWarning) {
+		// eslint-disable-next-line no-console
+		originalConsoleWarning = console.warn;
+		try {
+			// eslint-disable-next-line no-console
+			console.warn = filteredConsoleWarning;
+		} catch (error) {
+			// If we're unable to hook `console.warn`, just accept it
+		}
+	}
+}
+
+/**
+ * Indicates that the filter installed by `useConsoleFilter`
+ * is no longer needed by the calling code.
+ */
+function finishUsingConsoleFilter() {
+	consoleFilterRefs--;
+
+	// Note: Instead of reverting `console.warning` back to the original
+	// when the reference counter reaches 0, we leave our hook installed
+	// to prevent potential race conditions once `check` is made async
+}
+
+/**
+ * Hook/wrapper function for the global `console.warning` function.
+ *
+ * Ignores known non-problematic errors while any code is using the console filter.
+ * Otherwise, simply forwards all arguments to the original function.
+ */
+function filteredConsoleWarning(msg, ...rest) {
+	if (consoleFilterRefs > 0 && typeof msg === 'string') {
+		// Astro passes a `class` prop to the Svelte component, which
+		// outputs the following warning, which we can safely filter out.
+		const isKnownSvelteError = msg.includes(
+			"was created with unknown prop 'class'"
+		);
+		if (isKnownSvelteError) return;
+	}
+	originalConsoleWarning(msg, ...rest);
+}