init `partial` function and documentation

documentation/docs/06-runtime/04-imperative-component-api.md

@@ -81,3 +81,46 @@ const app = hydrate(App, {
 ```
 
 As with `mount`, effects will not run during `hydrate` — use `flushSync()` immediately afterwards if you need them to.
+
+## `partial`
+
+`partial` lets you create *partial components* — components composed of other components. 
+To best explain partial components, here's an example without them:
+```svelte
+<script>
+	import Greeter from './greeter.svelte';
+</script>
+<Greeter greeting="Hello" name="world" />
+<Greeter greeting="Hello" name="Earth" />
+```
+There's some clear repetition here; the `greeting` prop has the same value twice. Using `partial` we can make this much more concise:
+```svelte
+<script>
+	import { partial } from 'svelte';
+	import Greeter from './greeter.svelte';
+	const Hello = partial(Greeter, { greeting: 'Hello' });
+</script>
+<Hello name="world" />
+<Hello name="Earth" />
+```
+Snippets can be used with partial components easily:
+```svelte
+<script>
+	import { partial } from 'svelte';
+	import Paragraph from './paragraph.svelte';
+	const Example = partial(Paragraph, { example });
+</script>
+{#snippet example()}
+	According to all known laws
+	of aviation,
+	there is no way a bee
+	should be able to fly.
+	Its wings are too small to get
+	its fat little body off the ground.
+	The bee, of course, flies anyway
+	because bees don't care
+	what humans think is impossible.
+{/snippet}
+<Example />
+<Example />
+```

packages/svelte/src/index-client.js

@@ -1,5 +1,5 @@
 /** @import { ComponentContext, ComponentContextLegacy } from '#client' */
-/** @import { EventDispatcher } from './index.js' */
+/** @import { Component, EventDispatcher } from './index.js' */
 /** @import { NotFunction } from './internal/types.js' */
 import { untrack } from './internal/client/runtime.js';
 import { is_array } from './internal/shared/utils.js';
@@ -90,6 +90,60 @@ export function onDestroy(fn) {
 	onMount(() => () => untrack(fn));
 }
 
+/**
+ * Creates a partial component with the specified props.
+ * Example:
+ * ```svelte
+ * <script>
+ * 	import { partial } from 'svelte';
+ * 	import Greeter from './greeter.svelte';
+ *
+ * 	const Hello = partial(Greeter, { greeting: 'Hello' });
+ * </script>
+ *
+ * <Hello name="world" />
+ * <Hello name="Earth" />
+ * ```
+ * @template {Record<PropertyKey, any>} Props
+ * @param {Component<Props>} component
+ * @param {Partial<Props>} props
+ * @returns {Component<Partial<Props>>}
+ */
+export function partial(component, props) {
+	const initial_props = props;
+	return function (anchor, props) {
+		const merged = new Proxy(/** @type {Props} */ ({}), {
+			get(target, prop) {
+				if (prop in props) {
+					return props[prop];
+				}
+				return initial_props[prop];
+			},
+			set(target, prop, value) {
+				if (prop in props) {
+					//@ts-expect-error don't know why this won't work
+					props[prop] = value;
+				}
+				if (prop in initial_props) {
+					// @ts-expect-error ditto
+					initial_props[prop] = value;
+				}
+				return true;
+			},
+			ownKeys() {
+				const keys = Reflect.ownKeys(props);
+				for (const key of Reflect.ownKeys(initial_props)) {
+					if (!keys.includes(key)) {
+						keys.push(key);
+					}
+				}
+				return keys;
+			}
+		});
+		component(anchor, merged);
+	};
+}
+
 /**
  * @template [T=any]
  * @param {string} type

packages/svelte/src/index-server.js

@@ -33,6 +33,8 @@ export function unmount() {
 	e.lifecycle_function_unavailable('unmount');
 }
 
+export { partial } from './index-client.js';
+
 export async function tick() {}
 
 export { getAllContexts, getContext, hasContext, setContext } from './internal/server/context.js';